This all seems very familiar, and this is an older thread that y’all have restarted.
With iPhone, consider using Assistive Access. That might help here, but it does have its limits.
My most recent experience here was prior to the availability of Assistive Access. I’m unsure how much that will help; whether that can be another useful stop on the paths we each follow, or whether that might merely become another source of frustration.
Pragmatically, you’re probably headed for a feature phone though, and those do still exist. Nokia makes some models, and there are others. I did see some flip phones on offer, as well.
The feature phones also avoid having to block the ever-increasing piles of scams in the arriving mail and messages. iPhone and iPad filtering capabilities are unfortunately very limited.
Get iCloud backups going, if you are using iPhone. Stuff will get deleted, dropped, lost, or stolen.
Moving iPad to iCloud services for photos and backups and such too, if you’re also using iPad.
Get Legacy Contact configured, and Recovery Contact too.
Having tried to keep iPhone and iPad working for some folks with declining facilities, I’d now migrate to the feature phone much more quickly; sooner than I had previously tried. That and maybe a big-button home cordless phone, if they still have a landline.
The other difficulty that can arise here are loaned devices. Kids can and do “helpfully” delete or reconfigure all sorts of things. As was referenced elsewhere recently, a senior’s favorite game was simply deleted. Years of history gone.