How about make it so that the "hidden" photos on a phone require a security code/biometric to access? I've always been shocked that this isn't the case with iOS (don't know Android), it seems so obvious and simple.
It's fairly simple to use, and if you sometime give your phone to other people / kids / etc ... It quickly becomes absolutely necessary.
Need to remember to use the "secure folder" camera though, if you merely take the pic THEN move to secure folder, while it's super quick and easy it's usually too late as google photos, dropbox, whatever else will already have duped it.
Pixel with the latest Android should have that ("Move to Locked Folder" [1]), though as with all security things it is annoying to use in a lot of ways. Doesn't work for SMS images or Whatsapp (Signal is much nicer on this front, but images on Signal get lost if a phone is bricked - the account backup/transfer method sucks a bit).
[1] - https://support.google.com/photos/answer/10694388?hl=en