> The minute Google Photos was installed, it started backing up photos

Just to be clear: It will ask you before doing it.

If you refuse, it will ask you again and again and again. Sometimes with a slightly different prompt. Until you accidentally say yes.

But it does ask you.

Even though I agree with your overall conclusion that people should avoid google photos, this moment should also be a learning experience for your family to be more careful what they agree to. Popup fatigue is insidious, we all need to remain vigilant!