How to get better at classical chess when it's tough to play online

I've played chess online since COVID and have gotten pretty good at Rapid/Blitz (2000-2100 rapid and 1900-2000 blitz usually).

My kids have recently gotten into chess and I started playing in some OTB classical games.  I recently played in a mixed kids/adult tournament with them and did worse than I would have thought given my online rating  (lost 3/3 games to 1600/1700 players).  None of them beat me tactically in the opening or middle game, I just got into slightly worse end games.  I'm realizing that I'm good at quickly analyzing low-to-medium complexity tactics in complicated middle games when time is an issue, and that is why my rapid/blitz is so high.  My style has always been to sacrifice material for attacking positions and it works very often in rapid/blitz.  But when my opponents have plenty of time I can't quite convert those sacrifices and I end up in a worse end-game.

My question is - what would be the best way for me to practice for classical games given playing classical games online is not always feasible.  Any advice is greatly appreciated!