Giving an Android phone to elderly/non-technical people is asking for trouble imho. They will eventually tap their way into installing suspicious apps, adware or even straight up malware. It's inevitable, they are not aware of what they do and how to avoid the many risks of the digital world. I remember having the same struggles of OP when setting up a cheap android phone for my grandma, the amount of bloat, adware and misleading content I had to remove was incredible (and some couldn't even be removed). The irony was that after a few months of light usage, the phone was in a state even worse, full of downloaded apps and opened suspicious websites in the browser. She would swear she never even noticed any of those.
This is one of the cases in which giving them an iPhone with its walled garden has great benefits. You can also setup parental control on top of that already locked down ecosystem.
My mother can no longer do the stuff she used to on her iOS phone because it is so complicated compared to the iPhone 4 I gave her a long time ago.
I screen her emails with her consent, very easy to do with Fastmail that imports her Yahoo mail into a folder she doesn't see and then I move okay emails to her inbox.
If your relatives are significantly tech illiterate, I'd skip the smartphone entirely and go for a locked-down Linux desktop + feature phone. The most dangerous apps are big legitimate ones.
If you do go for a smartphone, my experience tells me that there's no difference between Android and iOS. The biggest sources for shady apps are the Google Play Store and Apple App Store. Shady stuff on the web can be easily defeated using an adblocking browser, which is essential for older relatives.
FYI: you can also set up parental controls on Android.
Parental control is a also a hot buggy mess on iOS currently. Our daughter has an iPhone with parental control set up and a bunch of apps that are whitelisted regularly refuse to start at random moments (blocked by parental controls). We hoped that iOS 26 would finally fix it, but nope.
It doesn't really matter, both phone ecosystems are a mess, but in different ways.
But I thought sideloading wasn't a real problem and Google is just locking it down because they're evil :-)
My mom has had several generations of Galaxy A5x and basic iPads, it makes absolutely no difference. She simply never installs any app.
Some things are actually worse on the iOS side. It took years for Apple to catch up with spam and scam calls/SMS detection.
Plain Google search is still the main vector of scams, I eventually set up NextDNS on her devices.