You can basically have this today - you might have to give up a few niceties, but if you don't rely on anything too fancy, Lineage + F-Droid, with optional MicroG will generally do the trick for most apps.
Gotta be a bit of tinkerer to set that one up though, not something for grandma.
If she had an iPhone, how many paid apps or in-app purchases was Grandma going to make anyway? Probably none.
Heck, I'm a developer and tech enthusiast that owns numerous Apple devices, and I honestly can't remember the last time I made a financial transaction through the iOS App Store or IAP.
Gotta be a bit of tinkerer to set that one up though, not something for grandma.