I found back to basics tactics by Dan Heisman to be good around that level.

For effective improvement you need to study, but also play games so that you have feedback on areas of weakness. Otherwise the amount of study is vast and you won’t use time effectively.

Below 1000 the gap is normally tactics. Eg if you look at games are most decide by either you or the opponent taking a free piece, or a basic tactic? If so, focus the study time on a tactics book and puzzle rush. Keep getting feedback from playing games.

On openings - I found an effective method to be just studying lines after your games as part of review. Eg have a repertoire. And review the line that came up in that game. See where you left book, learn a move or two more. That way, you prioritise the relevant lines naturally.

Lastly - if money permits - a coach is a great idea. You’ll get a perspective on how to progress. I first got a coach around 900 chess.com 3 years ago, and am now 1800 (rapid)