For the moment it's a Pi 4 + Hifiberry Digi+ Pro with Volumio software. Volumio provides a pretty decent web UI, automatic scanning of connected USB storage, Airplay (v1), and a Spotify Connect plugin. There are also other plugins but I haven't looked into them too closely.

I also liked Roon Bridge on the Pi (and felt the audio output was slightly improved, although made no effort to counter the placebo effect) but couldn't quite justify the cost.