I see where you're coming from and my (then 76 year old) Gran loved using my iPad mini, but she found the screen too small and a full sized iPad too heavy to hold. She did like her iPhone, but would still routinely send random messages to the wrong person.
She would've also found the jump from iOS 10 to iOS 11 confusing, as she did from XP to Windows 8.1.
I'm only posting this because I went down the exact route you mentioned for exactly the same reasons and it backfired spectacularly, and expensively.