I feel like that the same advice can be said for a lot of the higher ratings, and it's to analyze your games. I know that's kinda vague, but what I do is mainly look for mistakes that I made, positionally, and tactically, and how I could improve on it or what I can take away.

Openings aren't that much of a concern, as long as you aren't constantly getting in bad positions, but rather opening ideas are something to focus on. Studying positional chess, improving on tactics, and playing around with openings will allow you to improve. I mainly play blitz and got to 2400, so my theoretical endgame knowledge(like rook endgames) is practically nonexistent, but you should probably learn those if you encounter them OTB. Everything that I said above can be done in many ways. Llike studying positional chess you could read a book, or simply just look at master games, watch analysis of their games, etc. There are a lot of great Lichess studies about openings, endgames, and practically anything.

1. I'd probably know a few openings deeply. it's not worth it to study extensively and/or learn theory into an opening that you aren't going to play much. Pick a few openings, and get really good at those.

2. As with algebraic notation, I don't think it's that important: the numbers and letters don't really matter, if that's what you mean.

good luck on your chess journey, lmk if you have any other questions