My use case is scoring live events that may or may not have Internet connection. So normal usage is a single person but sometimes it would be nice to allow for multi person scoring without relying on centralized infrastructure.
Here's the app I built if you want to try it out: https://github.com/chr15m/watch-later
I've been writing a budget app for my wife and I and I've made it 100% free with 3rd party hosting:
* InstantDB free tier allows 1 dev. That's the remote sync.
* Netlify for the static hosting
* Free private gitlab ci/cd for running some email notification polling, basically a poor mans hosted cron.
IIUC, InstantDB is open source with a docker container you can run yourself, but at this point it's designed to run in a more cloud-like environment than I'd like. Last time I checked there was at least one open PR to make it easier to run in a different environment, but I haven't check in recently.
It seems like it'll be impossible without an overlay network (like Yggdrasil, i2p), but these will be too heavy for mobile devices without a dedicated functioning relay... here we go again.