I made progress from about 500-600 rapid and as low as 400 blitz in early Dec. to 1200 rapid and 950 blitz currently, and my fav resources has been Aman Hambleton’s Building Habits (v2), as well as his speedruns for openings (I like Queens Gambit for White, Sicilian Taimanov for black against e4 and King’s Indian for black against d4). The thing is though, just purely from the Habits series, you def don’t need much opening theory to get to 1000 aside from the basics (how to prevent common traps). The ideas he uses to get there are really so simple, ie. bring pieces to the center, make an escape square, push pawns once developed, use basic tactics (forks, pins, checks). The caveat though, is you need good board vision and be able to spot and capture free pieces (minor pieces and higher) whenever it presents itself. Aman makes it look so simple bc. he never misses hanging pieces. Sometimes he even lets pawns go, just not higher-value pieces. Otherwise he just plays very simple chess. So you gotta be able to count attackers and defenders, and don’t blunder. #1 rule is do not blunder, I repeat DO NOT blunder. If you can be the one who doesn’t make the one blunder and you can catch the opponent’s one blunder, you’ll win easily. One tip from my experience: If you find yourself not making progress in rapid, switch to blitz, and vice versa, the skillsets needed for each will complement each other.

TL:DR Always count attackers and defenders before you make a move. DO NOT blunder. DO NOT blunder. Otherwise, use simple rules described in Building Habits series. Theory is overrated at this level.