This might sound antithetical to the privacy-by-default standard, but my wife and I currently share our photo rolls with each other (using Google Photos)–everything we take is automatically shared with the other.
Is there anything in Proton Drive/Photos that would allow this? Even if it’s only available within a family plan or something. I know that with Drive, we could share a directory and everything in that would be available to the other, but I don’t see this as an option in the photos.
Alternatively, I suppose we could both sign in to Proton Drive using a third account just for that purpose, but that’s less ideal.
Syncthing.
Just sync your DCIM folders.
You can enable “Ignore Delete” on each phone if you want to ensure photos aren’t deleted on the source phone if the destination deletes it (or not).
I sync stuff to a PC at home, so phones don’t have to be on at the same time - they’ll always sync to the home PC.