Hey on android? Didn’t know simplemobiletools was sold. I use their qr scanner. Should be fine if updates are disabled from play store and only f-droid versions are used.

But for alternatives :

For a gallery app, I find Aves to look pretty good and has many great features github.com/deckerst/aves

If you need gallery (still not that great yet) & backup : ente gallery (there are other apps too) ente.io

For a calendar no idea.