“Sideloading” does not play a role in this. All of the crapware is on Google Play.

Funny, as someone that uses Android, sideloads apps, and is the "tech guy" for some older people, I went "yep, Google's own Play Store is full of shitty apps".

I recommend getting an Android phone (there are cheap Google Pixels out there) and try to sideload an app. Also browse the web a bit without an adblocker. I'd be surprised if by the end of the experiment you thought that sideloading is the reason their grandma's phone is full of crap.