Tried again with Squeezelite disabled. No matter what I enter in the „mixer control“ field, it doesn’t get saved, so no hardware volume control possible in Spotify Connect. Maybe „Master“ and „Digital“ are wrong for my Hifiberry Amp? I have no idea where to look for the correct mixer name.
(Off-topic, but I’m mentioning this because it just happened: When I switch to dmix output on one of my three synchronized Squeezelite players, all three start stuttering. Took me a while to find the problem because I had already forgotten that I switched to dmix.)