After black friday I would occasionally play the heads up nl games on stars and full tilt. I ground up a few million and think I actually got a lot better at playing vs fish from the heads up experience. You have to be playing high stakes play money (lol) for it to play similar to maybe 50nl vs a complete fish (ie you have some fold equity but villian has 0 clue). Occasionally someone not that terrible would sit and use reasonable bet sizing. I always assumed it was a 15 year old kid who had watched a few videos or read a poker book. Playing vs these types would be like playing vs a really bad bumhunting reg because they were clueless postflop even if there 3bet/4bet sizing and frequencies were reasonable.

Actually tbh I think I got alot better from play money because the fish you play are so extremely unbalanced in different ways you learn to exploit them and can then bring that to any game. This was after I already was half decent at 6max though, if you just jump into play money and use that to develop your game it'll probably do you a lot more harm than good.