For emulators:

• L4-R4 select save state slot, L5-R5 save/load
• Not back buttons, but: Select + A/B/X/Y for Ctrl-Q, F11, etc

In general:

• Press R4 to start pressing A every 500ms so e.g. it picks the next level and various confirmation popups automatically
• Press L4 once to keep another button pressed, e.g. R1 in racing games

I don’t remember any others off the top of my head, but the gist is that when I’m annoyed by some weird specific input I start fiddling with Steam input for a few minutes to try and make it work more easily 😅

I haven’t tried it, but I’m pretty sure I remember seeing a decky loader plugin for it!