[zaphar]

You haven't actually listed one yet. I can't think of one myself that Grandma could use safely.

[port11]

Duplicati was easy enough to convince the family. And for our home server, I set up Restic. I agree that this later one is tricky to set up properly and grandma would definitely struggle.

[dTal]

SyncThing. It's set-and-forget. All you need is two networked computers. Grandma and Grandpa can back up to each other's phones.