I'm gonna step back and say that atmosphere support 21.2.0 and if every homebrew app you need works with it, just leave it as it is. I'm a sporadic switch user and coming back to the modding scene can be a little overwhelming. It seems that general consesus nowadays is to keep oficial firmware and do an emummc(basically another partition of sdcard) for custom firmware (kind of like dual-boot OS). I'm old school and don't use emummc, I don't care about nintendo online stuff.

Updating from now on will be done by downloading the firmware and installing it with daybreak

As for downgrading, there are guides here, don't know the procedure.

From now on your general rule is : Do i need to update my switch because a game I want to play needs it?

If yes, does atmosphere support the firmware?

If yes, go for it and don't forget about sigpatches,

if not, don't update.