Calibre is one of the great pieces of FOSS software, and demonstrates everything good about FOSS: it has regular updates; it’s been around for simply ages; it works really, really well; it gets updates and new features and yet has never in my memory had a breaking, non-backwards-compatible release… it’s stable; and it resists - in its way - the attempt by publishers to steal our rights and ownerships of our media.
I contribute donate to Calibre. I hope that Goyal has a successor lined up to take the helm who can continue such an outstanding contribution when he finally retires from the project.
Edit: clarification
I use snapcast throughout my house and devices, but there’s no snap_server_ for Android.
I’ve been meaning to try roc, for which there is an Android client that will both play and serve.
Sonobus also claims to be many:many; I haven’t tried it either and it doesn’t look particularly active.
I don’t use UPnP or DLNA because of the security issues, so I can’t offer a suggestion about that. I thought DLNA was a pull oriented protocol - like, to send music from your phone you’d have to select and play on your computer with a DLNA client. Can you push media with DLNA?