D4 repertoire which consists of the catalan.

As somebody who very much thought about playing the catalan, yes, this sounds completely reasonable, as you should normally try to get similar positions in your repertoire (for example, if you play caro, you should prob play the slav) Against the KID, you could play the fianchetto variation (I personally love this against the kings indian!). Against the QID, you could play the fianchetto variation. Most importantly, have fun! If you dont like an opening, dont play it!

I dont know why I am even trying to give you suggestions tho, you are WAY outta my league. (Im literally only 850)

Edit: Just realized im stupid. You wouldnt be playing g3 against QID, as you probably would probably initiate the catalan with 3. g3 against east indian in general.

The Catalan only works against lines where the LSB is behind the pawn chain.

If you play the Catalan, you must know the Fianchetto Benoni (1.d4 Nf6 2.c4 e6 3.g3 c5 (instead of 3...d5).

Obviously the Fianchetto King's Indian, Fianchetto Grunfeld, and g6 against the Dutch are fine, but they are completely different openings and play nothing like a Catalan.

Where the Catalan setup is bad is lines where Black's LSB is outside the pawn chain, like in Slav setups. A few GMs have played g3 against the Slav, but it is significantly weaker than g3 in lines where the B is behind the pawn chain, and is why 3.Nf3/4.Nc3, 3.Nf3/4.e3, and 3.Nc3/4.e3 are all more common.

The Catalan is basically an opening to answer attempts at the Nimzo-Indian and Queen's Gambit Declined.

Fianchetto systems also don't work very well against the QGA (1.d4 d5 2.c4 dxc4 3.g3?!)

Perhaps the repertoire book Keep It Simple 1 d4 would suit you? That book uses the move order 1 d4 2 Nf3 3 g3 4 Bg2 5 0-0 6 c4. Delaying c4 allows white to avoid a few black defenses. However, there are times when the move order is derailed, such as when black plays an early ...c5 for example. The author has repertoires for when that happens.

you can.

but it reminded of the story about a construction worker who only knew how to use a hammer.

he was great at framing houses but his glass work was a little suspect.

Could you please tell me what approach the author has taken against other lines?