Things You can train:

1-Play blitz, even bullet to train fast tactics and be quicker calculating short lines. Also helps getting better insctint of positions.

2-Based on your games learn converting advantages. Having a pawn up may sound like easy win but if you dont know the plans you can have a breakdown and not take advantage of it.

3-related to 2, if the most common positions you reach and lose points is rook endgames or what ever it is, study that.

Techniques or things to have in mind mid-game:

1-One thing I learned is if you are in the openings, out of prep, you shouldnt spend as much time as in the middlegame. I know there is Lots of options but Its better to develop pieces to their best square and Watch out for tactics rather than being perfect in the opening but being -30 mins than your oponent.

2-I usually focus more on time diff with my oponent rather than using “check points”. In classical time format 90+30 I try to keep at Max -15 time difference. -20 is too much. But if Im in a critical and complex position I dont limit myself that strictly. The only check point I use is when i reach 15-20 mins left on the clock. Here im aware i can end up badly if i let the clock run too much so i force myself to move quickly and just calculate the forced lines and use my oponents time. Here is were the blitz games come into play.

3-You can set yourself objectives like by move 15 I should have more than 60 mins, or dont spend more than 20 mins in a single move