While I was researching I found out about Squeezebox, as there are people using it in combination with HomeAssistant. Both solutions you and @cfi provided seem pretty doable, and I've already been tinkering with Mopidy on armbian. Snapcast is something I've never heard of, and I'm definetly going to tinker around it, I'd love to be able to sync several speakers around the house, specially for parties and gatherings.
That being said I think they are a bit overkill for the usecase, and I'm looking for something even simpler, maybe repurposing the guts of a cheap BT speaker I have lying around, see if I can find somewhere on the PCB where I can tap line level audio output and solder it directly inside the amp/sub box, along with a small power supply to run without batteries. (I know there are ready-made BT modules for this, but where's the fun in that!)