The most secure way that I have found (for me) is to use proton drive on each user account. photos can be set to backup and you can move files into the drive app from your files app and it will sync
Sharing files between user profiles?
Proton has been... unreliable... about actually open-sourcing their apps, and keeping their open-sourced files up to date so people can build the apps themselves to double-check Proton and make sure they aren't pulling any funny business.
They also have been caught pinging google... Though to be fair, it is for reasons and within contexts I don't claim to understand, but that looks like a red flag to me.
Maybe these errors are good-faith mistakes, maybe they aren't. I personally am migrating to Tuta email and calendar.
I intend to use syncthing-fork for the rare occasions I need to move stuff between profiles, and for backups, when I get around to all that.