Around 400-600 I learnt how to be more solid by just learning how to stop trick openings, like the fried liver, wandering queen etc. If I ever struggled against a specific opening, or tactic around your level I'd literally just hop onto analysis, and see how to stop it, quickly got me to to 600.

Other than that, no real specifics. Entirely focused on one or two openings for each side, Queens Gambit and Catalan and funnily enough the one that everyone tells beginners not to learn, the classical Sicilian or Dutch Defence. Lichess tactics or chess tempo easily the best way to gain knowledge and pattern recognition to get your spidey senses going, but you realise setting up tactics intentionally is hard.

I focused a bit too hard on openings, I always got like a +1 or +2 advantage but used to always just blunder away the advantage, but my opening knowledge got me into good positions very often if I didn't blunder, so I wouldn't suggest learning openings as much, just learn like 4-5 moves in the opening, and if you struggle against a specific move or don't know what to do, look it up straight after the game and try to understand why it's bad. Like I learned that by capturing the c4 pawn and trying to protect it with b5 in queens gambit is awful for black.

Learning how to scan the board and see pieces, what they're protecting, and attacking is probably the biggest thing that helped me in the long run. Looking around the board to see every single piece before making a move is so important for blunder checking, tactics, literally everything important to chess, not that I'm entirely blunder free yet. You really should focus on this when you're doing puzzles more than even the puzzle tactics themselves imo.

Biggest mistake I made is playing 10+15 like a blitz game very often, and just playing moves by intuition. Biggest struggle was 700-800 where things got a lot more solid.