I would trash the Budapest. It is not about playing "main lines" as much as it is "sound lines". You want openings that are absolutely sound when playing top level players.
Sound and main lines are not the same thing.
The Vienna Game (1.e4 e5 2.Nc3 Nf6 3.f4, 1.e4 e5 2.Nc3 Nf6 3.Bc4, 1.e4 e5 2.Nc3 Nf6 3.g3, 1.e4 e5 2.Nc3 Nc6 3.Bc4) is Sound. Not the case with the Vienna Gambit (1.e4 e5 2.Nc3 Nc6 3.f4) or the dubious 3.g3 line against 2...Nc6.
Anti-Sicilians - There is nothing wrong with the Rossolimo, Moscow, Alapin, Closed, Grand Prix, etc. DO NOT go for the Wing Gambit or Morra Gambit at that level.
Queen Pawn Openings - Nothing wrong with going 1.d4/2.c4. If you are going to play openings like the Colle System, London System, Torre Attack, Trompowsky Attack, etc, DO NOT resort to one opening and think it can be played against anything. For example, the London is no good against the Modern Defense. The Torre is no good against 1...d5 or 1...Nf6/...d5 (because of 3...Ne4!, which is not the same as the Trompowsky because White has played 2.Nf3, and so no way to harass the Knight with f3). The Colle is no good if the LSB is developed by Black OUTSIDE the pawn chain. 1.d4 d5 2.Nf3 Nf6 3.e3 Bf5 (or 3...Bg4) and White MUST play 4.c4, leading usually to a Slow Slav. The Trompowsky only really works against 1...Nf6.
But if you combine them, like Torre against 1...Nf6/2...g6 and 1...Nf6/2...e6, Colle against 1...d5/2...Nf6/3...e6, and Offering a Transposition to the Slav against 3...Bf5 or 3...Bg4, you'd be good if you want to avoid main lines.
Flank Openings - Again, anything that is SOUND. Does not need to be +0.2 advantage. As long as you keep at least Equality, you are good. So ones like the English, Reti, King's Indian Attack, Sokolsky, 1.Nc3, Larsen's Opening, etc are fine. Bird's Opening is very borderline. Avoid garbage like 1.f3, 1.e4/2.Ke2, Grob, Sodium Attack, etc.
Black vs e4 - Basically here you want to play any one of the "Big 4". Double King-Pawn, Sicilian, French, or Caro-Kann. Whichever of those 4 you specifically are most comfortable with, go for it! For me that's the French, but for someone else, it's the Sicilian, and for someone else, it's the Caro-Kann, or the Petroff, or the Spanish.
Black vs d4 - QGA, QGD, Slav, Nimzo-Indian, Queen's Indian, Bogo-Indian, King's Indian, Grunfeld, Dutch - Avoid the garbage like the Clarendon Court or Albin and also avoid the dubious lines like the Snake Benoni or the Budapest
Hope this helps. And I speak as someone that has been USCF-rated between 1930 and 2185 for more than the last 20 years. So I face 2100 opposition a lot!
So I'm currently 2100 rapid with a peak of 2158, and I'm wondering if I should start playing mainlines rather than sidelines.
I've been playing Chess for almost 2 years now, and the entire time I have done so I have pretty much exclusively played sidelines. As I have grown from 800 to 2100, my openings have changed accordingly, but one thing that hasn't changed has been my avoidance of mainlines. For example, let me list some examples.
I play the Vienna Game, which occurs in less than 7% of games in that position, the Caro-Kann Defense in less than 11% of games, the Alapin Sicilian in less than 6% of games, and the Budapest Defense in less than 7% of games. These and other unlisted examples make up the vast majority of my games.
With all of these I do get decent success, but I think a part of that is they are relatively uncommon for my opponents to play against, and as a result they do not know what to do a lot of the time. As I get better and better, my opponents are starting to know a lot more theory for these, and I lose my element of surprise, which is a chunk of why I do well with these uncommon openings.
So should I continue playing my sidelines and rare-ish openings, or should I start to convert to the objectively best mainlines as I continue to climb the rating ladder?