My parents are tech illiterate, and very scared of being scammed. When I mention like “you can listen to any song you like, amongst millions” because I set up family sharing on their iphones, and ask them to use Apple Music they ask what does it look like. I then send them screenshots of my screen where I point out what Apple Music app icon looks like. Then in a few weeks I have a video call with them where I walk them through the interface.

I. I do want to be able restrict what they can install on their phones and it gives me a small peace of mind that nothing they install would outright steal their bank app’s credentials or all their passwords.

I think GP is fair when they don't want to support crazy sideloaded app land. We all live by the same rules, and I also would like to be able to install anything, but supporting my family is super hard as it is. It's about removing land mines, not trying to prevent people from doing what they want. I've had three separate relatives get their accounts hacked in the last month. Think of what an unbridled app store full of scammy apps is capable of, and then think about trying to support non-technical people through that.

If they have an expectation of me spending time to maintain, cleanup, and update their devices, yes I would like to have some say in minimizing the problems they can create.

And in the same way you can use website blockers to prevent them from accessing phishing sites, or give limited user accounts on a computer, iOS has parental control rules you can setup. It already has a feature to prevent installing (and deleting) apps.

https://support.apple.com/en-us/HT201304

My buddy's a general contractor. He's helped me fix things around the house. If I called him because my front door won't close, and he arrives to find I've taken a chainsaw to it, he's going to chew me out. It's not fascism for him to tell me "don't carve up your house if you expect me to fix it".

Sure, but would you be happy with your buddy installing special limiting power plugs in your home which prevents you from ever powering any technical tool ? That's exactly what is happening here.