Value betting, maximizing bet sizing and knowing which players and board textures allow more the greatest elasticity in the upward ranges like 90% pot, pot, overbets, overshoves, etc
Don't ever think about GTO ever. Seriously. At the time you think you need, you don't. Then the next time you think you need it, you don't. 1kNL players need it. Not you. You want to study exploitative play.
Keep it simple. I have never met a good LAG...ever. They probably exist at 200NL+ but not any tables i have ever played at. When I was new I thought LAGS were the best ever but really now I see they just suck and are more annoying than good.
Don't play more than 6 tables, preferably 4 so you can focus on making the most perfect decision on every street. Bet size, think about ranges, check out villain's HUDs when not in a hand, make very detailed notes, reorganize your notes on every villain, recheck HUDs when not in a hand, develop a game plan vs every villain at all 4 tables and just keep thinking...quality over quantity
Consider a coach. PM me if you are interested.
also this
https://www.youtube.com/user/thepokerbank
watch all the videos multiple times if you are actually a true beginner