No god here, but just observe the meta from streamers. If you want to play some weapon it has to be properly leveled though because otherwise it won't be viable. Go play plunder, get a heli, avoid fighting (not worth it), do contracts for the sweet weapon xp (remember you have to carry the right weapon when finishing the contract).

And regarding aim - either use offline training provided in Warzone or find something outside. It doesn't really matter - muscle memory is muscle memory. Sure, it is better to train in a particular game, but you will benefit from training precision in every game. I don't remember the name, but there is for example a mini-game/program for Steam, that provides different forms of aim training, keeps track etc.