@neil I'm a huge fan of Roon:
I've been using it for a few years (migrated from a SlimServer setup). Roon has a really nice user interface, has iOS and Android and desktop apps, the server runs on Linux (optimally, although Windows and MacOS are also options), and a supercool premium feature Roon offers is bringing in extensive metadata for all the existing music of yours it identifies — you're not limited to your own tagging.
@neil Now, #RoonLabs caveats relative to your initial wishlist:
There is some communication with Roon's central servers for the purpose of maintaining functionality and doing the necessary queries to pull your music's metadata.
There are dedicated Roon desktop programs for macOS and Windows, but no production-quality web interface.
The Linux server expects to be running on something Intel/AMD amd64ish.
There do exist free builds for Pi streaming audio endpoints!
Jeff Moore