Personally, I would find it very challenging to justify buying an iPhone 16e over a used iPhone 15 Pro that can generally be had for not much more. I know a lot of (“normal”) people who get a lot of use out of Magsafe tbh, and simple point-and-shoot photos often benefit from good zoom. But if you really want to buy new or directly from Apple I don’t think you can go wrong with the 16e or any of their current models either.
Stock Android is the worst operating system on the market from a privacy perspective, and I don’t even consider it a close race. I would never buy someone a Pixel unless I knew they would use a custom ROM. I do think GOS could potentially easily work for the use-case you’ve described, without her even knowing about it, but I also think it is valid to not want to experiment on your mother with this sort of thing 
If you trust the GOS team (which it sounds like), then GOS agrees that an “iPhone is the next best choice for a private and secure smartphone” after GOS.
Anecdotally, I can confirm that every older, lifelong Android user in my family who later switched to iOS has needed tech support from me significantly less often (essentially never) 
As far as iOS features go, I do not think your mom would benefit from Lockdown Mode, but she certainly would benefit from Advanced Data Protection. Using NextDNS can become a bit of a tech support problem, but if you’re fine with being tech support then it’s a good idea. Regardless of whether you use NextDNS or not, I would definitely install AdGuard to block ads both in Safari and in most in-app WebViews.
YouTube Premium is the only subscription service I find provides more value than it costs. Virtually infinite ad-free content on every topic imaginable for <$14 per month is crazy.