I play live 2/2 and the rake is high of course.

Should I learn GTO preflop and adjust from there?

In my country the skill level is decent, open size is 3x (6€) every time because it is not possible to 2.5x and 2x is too small because of equal blinds, 4x too much. BTN or SB might 4x sometimes. Open limping or odd sizes are very rare so it is not a pool full of drunk fishes.

GTO would be 3bet a lot because of high rake postflop and zero rake preflop and play pretty tight.

If weak player opens I can call/value 3b lighter to get postflop with the weak player.

Blind Vs blind against weak player I should be checking 100% I guess.

GTO 3bet sizes from the blinds are very high when open size is 3bb and playing deep, should I be following this strategy? I don't see the good regs 3betting as large as GTO. From bb GTOwiz recommends 5x and regs always 4x it.

So learn GTO and play GTO ranges against regs, against weak players play little bit wider range to get to the flop with them?

It helps to know GTO ranges as a base line and go from there. IE, when open limpers start raising, we should be pretty tight. Against players that call too much vs 3bets, we should 3bet less hands like suited connectors, QJs, JTs, and more hands like AQo, KQo, maybe AJo.

The GTO 3bet size vs a 3x open raise in GTO Wizard is 9.5bb from IP, 10bb on button, 13bb from SB, 14BB from BB. 5x is more when we open raise 2x at 100bb.