volume buttons seems disabled

if you can't solve your problem, you can add volume tiles to notifications: MediaVolume (Quicksettings Tiles for media volume) - https://f-droid.org/app/cn.morfans.chenjunyu19.mediavolume

Same on a S21. Guess it's a Samsung issue. Can't get it to work at all.

Would be nice for a comment Navigation panel at the bottom of the screen too

Same on my a51.

A workaround that works for me is to pull down the top menu by swiping down from the top of your screen. The audio buttons work when the menu is down. You can push the menu back up swiping up from the bottom of your screen or simply pressing the back-button.

Does your app have some setting that uses volume keys, for example navigation or similar? Might help checking the settings, and disable anything that suggests using volume buttons for something other than volume.