Awesome work. I'd highly recommend NIP-98, if you at all get a chance to look at it. It's just an auth header to a server, and it scales really well. The extensions can already sign events, so it's easy. NIP07 is used in a bunch of places, and I use it in nosdav for personal storage too. I see this pattern scaling to billions of users.

Nosdav

Cloud storage for Nostr

This little shim contains all the code https://www.npmjs.com/package/nosdav-shim