Make sure you do defensive puzzles as well. I'm fairly decent at spotting a tactic to win material a few moves ahead, but terrible at seeing one coming from my opponent early enough to avert it. I'm not sure what lichess has, but definitely look for puzzles where you have to avoid a knight fork or some pin or incoming checkmate.

Puzzles are great for learning tactics, but eventually you need to learn some strategy and positional play. You'll need to read a book or two for this. And don't forget endings!

And don't forget to study end games too. I don't think you need a strong grasp of opening theory until you reach around 2000 (lichess).

Learn end games before opening s. Learn k+q against a k. Learn k+pawn against k (how to advance a pawn to promote to q). K+r against K. There are others but get those 3 down.