Maintainer has been absent for some time so kernel v6.11 and v6.12 isn’t supported OOTB, to get it to work with kernel v6.11 you need to pull the fix from: !48

Additionally you can try and force use amdgpu rather than radeon, by setting the kernel flags:

radeon.cik_support=0 radeon.si_support=0 amdgpu.cik_support=1 amdgpu.si_support=1 amdgpu.dc=1

Source

Device initalization failed according to the Xorg logs;

1. Dump your firmware version
2. Dump your kernel version
3. Dump your kernel logs (dmesg or journalctl -k)

Running: swaymsg for_window "[app_id=mpv] opacity 0.5"

Works as expected on my end, are you missing just executing for_window?

Note, you can also add multiple rules in the same execution, e.g.

for_window {
    [app_id=mpv] opacity 0.85
    [app_id=LibreWolf] opacity 0.85
}

Also, note that app_id of LibreWolf is capitalized in that manner. You can get that information [app_id, shell etc] by running swaymsg -t get_tree