Typically L1/R1 shoulder buttons or L3/R3 stick click buttons since those are difficult for me. The rear L4/R4 ones are easier to use. Depends on the game and which I am using the most.

R5 I frequently map to take screenshots depending on the game. Except in emulators R5 is fast forward. The rest of emulator settings are a left touchpad menu that I set up almost identically visually for all emulators as possible so it’s easier for me to hop between systems.

For more complex games, typically older ones where I have to get super creative (like Sacred 2), I use the back buttons totally differently as gameplay needs it and as is comfortable.

Sometimes I find the custom control tinkering as much fun as the game. Trying to find a setup that works for me, tweaking as I go for the first few hours of gameplay, and eventually I save as a template with detailed info in it so I never lose it. I use templates for saving because the other save options are less reliable.