The one thing that stops me from using something other than Google Photos is that I use GPhotos for sharing with others, and I don't want people to have to sign up for a new account just to join albums, and I don't want them to have to install a new app in order to add photos to shared albums. I just don't think that's going to happen.

Looks like Immich supports OAuth, so presumably people could sign into my instance with their Google Account credentials. But then does the mobile app start using my instance to upload their photos, even if it's unrelated to an album I've shared with them? Kinda confused how this is supposed to work.