Pedantry alert: As ELO ratings follow a logarithmic curve, "gaining 600 points" is a dimensionless metric.
These are good tips for beginner to intermediate growth. The things that definitely help the most are:
* Pattern recognition - the best courses for this level are things like "Common traps in ", applied with Woodpecker method. Once you've memorized all the mistakes in the Scandi or London system, you can really crush a lot of people who play haphazardly.
* Study your own games and games of people at or just above your level. Four simple methods:
1) during the game, write down (Lichess has a notes section on the left) 3 candidate moves for every move in the middle and endgame, why you're making a particular move, and what you think the opponent's response will be
2) use the "Learn from your mistakes" button after each game during analysis
3) check the most common moves in the opening that are different than yours, play through a couple of masters' games to see why those positions are preferred.
And my last tip which helped me a lot just with the "meta" of playing chess ...
* Use more time. Be okay with losing games because you run out of time thinking. Always, always, always try to play the best move, even if it means spending a lot of time.