My advice is a little different than others. I recently got my in-laws outfitted with smartphones and a new nuc. My father in-law has Parkinson's, slow onset, but it means he is clumsy. It's not likely that his phone will survive shaky hands for 8 years. What I did was buy refurbished OnePlus phones, install a simple launcher, install Bitdefender, and then add a lock app that let's me add a pin to the system settings app and the play store.
They can't install anything out change things. They can browse, play games I preload, take pictures, etc, but I don't have to worry about them installing things they shouldn't. If one of the phone dies I'll just get the latest affordable refurbished of the same line and configure it the same way.
I had originally tried without locking the phone down as much but my father in law could not stop installing spammy weather apps and clicking ads on games and following there instructions.
I also created new Google accounts for them that I manage, so they can't get stuff stolen with bad account management practice.
If you are in the US, I've also found mint to be a good deal for cell service.