Same here. I rescently switched from ESPhome muze lux which was very buggy and unstable.
Now I use just like you the Slim proto HA integrations which seems to work much more stable in streaming music from HA (local files or radio streams or TTS). Further I can also use it standalone as a bluetooth or Airplay speaker.
I can t stream with my (free) spotify HA integration to the squeezelite muze luxe.
I can not also like you use the buttons: volume up down or play pause. Do I have to add something in the webpage settings ? In the NVM editor ? I installed my firmware with a web installer for ESP32 squeezelite where I could select the muze luxe so I was in understanding that all GPIO settings and DAC, I2S was preset in the firmware and that buttons, lights would work.
If you press the 3 buttons you can see an Event on HA on the Slim proto HA integration. So you could capture these in HA and for ex manipulate the volume,play pause through a HA automation. I was thinking these buttons would work outside if the box on the muze luxe squeezelite firmware. Am I missing some additional config ?