If you stick to Flatpak apps you don’t even need the codecs on your base system (I’ve been using it myself this way for a while now). For power management, I personally prefer to layer powertop, which doesn’t break power-profiles-daemon and works basically just as well as tlp, but layering tlp is perfectly fine too.

The custom kernel though, that’s more complicated (and an understandable limitation to immutability), I’d recommend you look into Universal Blue in your case, as it might be a better solution.