I don't have a systematic approach, but I found below video of Nelson Lopez very helpul. In the early 1000 (and below 1000), making a blunder is the most common reason losing a game. I recommend the "blunder checking" method!
Structured learning - 1000 to 1800
You're in luck, ive been searching for something like this fot the past couple days.
As far as books go:
Play winning chess by Yasser is 1000-1600 (as mentioned in the book) which can be followed up by winning tactics similarly by Yasser and likewise winning openings.
Build up your chess/boost your chess/chess evolution by Yusupov. There's some debate on what elo it's appropriate for, some 2000s say they struggle with exercises in the first book which was designed initially for 1500s or so. Ive been through some of the earlier chapters and what ive extracted has been awfully useful so far, no idea for the latest chapters.
For videos/courses:
Judit polgars chess course is supposedly good although I've not purchased it myself.
Chessfactor is free and wonderful all the same. It covers about everything you'd expect to learn I'd wager
#1
"systematic structured learning"
Play a game. If you lose it, then analyse it so as to learn from your mistake. This is the essence. With this alone AlphaZero got to +3000 without any other input than the rules.
Books are helpful, but you cannot learn to swim or drive a car by study of books.
Good books are:
"Chess Fundamentals" - Capablanca
"Bobby Fischer Teaches Chess" - Fischer
Another viable and systematic approach is to study
first 3 men positions: KQ vs. K, KR vs. K, KP vs. K,
then 4 men: KBB vs. K, KBN vs. K, KQ vs. KR, KQ vs. KB, KQ vs. KN, KQ vs. KP, KR vs. KB, KR vs. KN, KR vs. KP, KB vs. KP, KN vs. KP, KP vs. KP,
then 5 men: KNN vs. KP, KQP vs. KQ, KRP vs. KR, KBP vs. KB, KBP vs. KN, KNP vs. KN, KPP vs. KP

Everything else I've seen is an unstructured approach that involves 'learn openings', 'learn end game', 'study tactics'.
Learn openings - no. At the 1000 Elo level, openings have very marginal benefits, if any at all.
Learn endgames - you can, if you feel like it. It will improve your chess. But don't expect it to have a great impact on your results. Most games don't reach the endgame.
Tactics - yes, yes, yes. Solve a tactics puzzle (or up to 5, that is the free limit on chess.com) every day and you will improve a lot.

So right now I am following the courses, as described on the Chess King website:
http://blog.chessking.com/guide/
They have it broken down according to ELO ranges. I'll let you know if it works for me!!
Agree with #3. Yusupov's series (the first three books) will teach you everything you need to improve. Tactics, strategy, piece play, combinations, motifs, basic endgames and so on. It's great if you put in the work.

Well, I've always been told to start learning ENDGAMES. So thats my advise.
The book that I've found more suitable is "Complete Endgame Course" by Jeremy Silman. He divided endgame topics by rating. For example:
<1000 Rating: Opposition
<1200 Rating: Lucena / Philidor Position
You should give it a try.
Afterwards, you could start reading the first book of Yusupov series called: "Build up your chess 1".
I don't think there's a structured guide on how to get to 1800 from 1000. One reason for that that I can come up with is that 1800s are not so similar to one another. 1800s have big gaps in their chess understanding. There is no set amount of knowledge that they have that you can learn to be as good as they are. For example, when I was 1800 I regularly couldn't figure out how to win rook endgames where I was up 2 pawns.
What you'll notice when you're improving from 1000 to 1800 is that players start making less and less tactical mistake. 1800s will still make tactical mistakes, but it's not at all as often as a 1000 player. Getting better at tactics is the way to improve. Getting good at tactics really does take time and effort though. And you also need to get good at spotting tactics in your games, which is usually a bit harder than spotting them in puzzles.
The other thing I would always focus on as a chess trainer with my pupils (who I guided through this transition of <1000s to 1700s and beyond) is to teach them middlegame strategy. It's important to understand what a 'normal' move is. This is another part that 1800s will be better than a 1000 player, although they will definitely be making plenty of mistakes still. I have always used Silman's How to Reassess your Chess. Focus on the static advantages first, don't worry about the dynamic ones until later when you've fully mastered the static ones and gotten a higher raing. Basically you can simplify chess to 'if there is a tactic, I'll spot it; if there's no tactic, I'll make a normal looking move.' The better you are at this, the higher your rating will be. All the advanced stuff will start becoming important much later.
So that's my suggestion: tactics and middlegame strategy. I would only add the necessary endgame theory when you get to the point where you start to notice that you reach the endgame more frequently with maybe only a pawn or maybe 2 advantage or disadvantage. Then start on king and pawn versus king (how to win that and how to defend that) and some basic pawn endgames like outside passed pawns. Much later still you can try and look at the rook endgames: 6th rank defense or basic ideas like being active in a rook endgame. As long as you play with all your pieces, including your king, in the endgame, you'll likely won't get too many problems for a long time still.

Is there a systematic structured learning for improving rating from ~1000 to ~1800? Is that simply signing up for Chess.com Diamond and following all of their materials and studying content?
I recall seeing a list of books in recommended sequential order of reading to get the reader to 1800. Everything else I've seen is an unstructured approach that involves 'learn openings', 'learn end game', 'study tactics'. Perhaps, what I'm mostly intrigued in is a 'course' or syllabus that covers what content you should know at each and every level.
I understand the basics and the 'fundamentals', but want to truly escalate and bring to the next level. Would welcome thoughts on this subject.
Yes there is!
Run, don't walk, to get Dan Heisman's A Guide to Chess Improvement. The book doesn't try to teach you chess, rather, the point of the book is to teach you how to improve. And, in fact, the first two sections of chapter 1 (each about 10 pages) are entitled "The Theory of Chess Improvement" and "An Improvement Plan". The book gives you (among other things) a big picture view of how to go about getting better.
I would say that most (but definitely not all) of the advice here on chess.com is pretty good. But: (a) it's hard to separate the good from the not-so-good; and (b) even the good advice doesn't necessarily give you a context as to why it's good.
Dan Heisman also has a website (with recommended books -- listed according to strength), a twitter feed, a youtube channel, etc. But the book puts it all together really well. Heisman first came to fame as an instructor when he started writing the Novice Nook column for chesscafe. Many of the NN's are behind a paywall, but the first 8-10 years' worth are archived and can be read at https://web.archive.org/web/20140714190611/http://www.chesscafe.com/archives/archives.htm
The book which I am recommending is a compendium of his best columns, put into a logical/thematic order. I think that every player U1600 would benefit a lot from the book.
(I see that @tygxc above wrote: "Play a game. If you lose it, then analyse it so as to learn from your mistake. This is the essence.". This is, indeed, one of the four main things to do to improve, as described by Heisman)

If I remember right, I went to 1500 basically on tactics. Sharpen Your Tactics by Lein and Archangelsky, the Polgar tome, those were basically what I was consuming. I also really liked Silman's Complete Book of Chess Strategy just to learn a couple ideas from openings (which didn't end up terribly useful), some basic positional ideas (which I didn't appreciate at the time), and some basic endgames (I did retain a little bit on rook endgames that ended up useful).
1500 to 1600 was basically just learning how to take my time and at least look for anything that wasn't the first idea that came to mind. I read Soltis's How to Choose a Chess Move and basically learned a little bit about how to think and how to take my time.
1600 to 1850 has been a series of ideas: I read through Secrets of Chess Training by Dvoretsky and learned a little bit about how to study other chess games and how to analyze my own. I read the first book of Pachman's Complete Chess Strategy (the three volume set, not the condensed version published by Ishi Press) and learned some positional ideas. I started grinding tactics against with Woodpecker Method (currently working slowly through intermediate tactics). I also learned some basic endgames through Jesus de la Villa's 100 Endgames book. All my learning has been pretty scattershot and has several holes (there's no way I can draw rook and f/h pawns v. rook, ever), but I'm just working on patching them up.
I'd also recommend tactics and middlegame strategy. Endgame ideas are also nice, but like RAU said, maybe not too relevant yet. You likely won't be trying to convert Lucena positions or hold Philidor positions until ... eh, maybe 1600? Always useful to know, but I'm guessing it won't win you that many games yet.
I still haven't found opening knowledge useful aside from basic principles.
EDIT: One more thing: probably 50 of those rating points I can attribute to reading the first chapter of Smerdon's Chess Swindler. Not giving up in losing positions, and actively trying to set traps, has led me to convert a fair amount of games.

Level 1 is for up to rating 800 USCF
Level 2 is for up to rating 1400
Level 3 is for up to rating 1600
Level 4 is for up to rating 1750
Level 5 is for up to rating 1900
Level 6 is for up to rating 2100
Level 1 is mostly learning the rules, pieces, and board.
Levels 2 and 3 are almost entirely tactics with a few endgames and maybe a little bit of opening principles.
Levels 4 and 5 have some strategy, more endgames, and opening principles, but still mostly tactics.
Level 6 seems more balanced across subjects (tactics, attack, endgames, strategy).
Yusupov’s books are much harder, in spite of his own claims. Levels 1-4 of Steps method (which I have gone through) are all easier than Yusupov’s lowest level of training books, in my opinion. Level 5 of Steps (what I have gotten through) was difficult for me, but still seemed either easier or similar to the lowest level of Yusupov’s books (Orange), but I didn’t go through much of Yusupov’s Orange books when I saw the problems were too difficult.

Improving Your Chess - Resources for Beginners and Beyond...
https://www.chess.com/blog/RussBell/improving-your-chess-resources-for-beginners-and-beyond
Anyone else used the Chess Steps series mentioned above?
Yes! I have mentioned the series a few times. I think they are the best tactical workbooks and manuals out there. We all used to use them in Dutch clubs for training our youth.

Tactics, tactics, tactics.
Game 8 of the FIDE Championships (https://www.chess.com/events/2021-fide-world-chess-championship/08/Carlsen_Magnus-Nepomniachtchi_Ian) turned on Nepo's blunder of 21...b5.
At the press conference he was asked why he played that move -- didn't he see 22 Qa3+ as a response followed by 23 Qxa7? He said, of course he did, but he had forgotten that his bishop on d5 was hanging.
In other words, even players like Nepo make simple tactics errors which cost them games.
Is there a systematic structured learning for improving rating from ~1000 to ~1800? Is that simply signing up for Chess.com Diamond and following all of their materials and studying content?
I recall seeing a list of books in recommended sequential order of reading to get the reader to 1800. Everything else I've seen is an unstructured approach that involves 'learn openings', 'learn end game', 'study tactics'. Perhaps, what I'm mostly intrigued in is a 'course' or syllabus that covers what content you should know at each and every level.
I understand the basics and the 'fundamentals', but want to truly escalate and bring to the next level. Would welcome thoughts on this subject.