I don't have any internal data, but I expect that the percentage of Sonos users that play music from a local source is in the single-digit percentage now. The product they sell is focused on supporting streaming audio and voice control, all of which involved 100% monitoring of what you play and ask for. You're just not their customer anymore.

You can find old used squeezeboxes on Ebay, and their server software is just a big perl script. That might meet your needs.