I use the back buttons for all sorts of stuff. But the most common use is remapping ABXY to them. That way, I dont have to lift my thumb off the joysticks as much.
Also some games have easier menu navigation with a mouse, so I’ll map left and right click to r1 and l1 and use a track pad to navigate menus.
From what I’ve seen about foveated rendering, I don’t think developers need to do anything. It should just be “on” for any streamed content