Another solution that is surprisingly powerful is Logitech Media Server [0], which despite its name is open source and cross-platform. The server can run on any unix-ish machine, and clients can be any number of Raspberry Pis or ESP32s, or custom boxes, or any computer. Multiroom syncing is great. Works for local library and streaming. There is even a modern web interface available [1]. I looked into many of the other solutions in this thread, and LMS suited my needs best.