>I'd like to see regulation that forces Apple and Google to allow any and all app stores on their devices, and the process of installing apps through those stores must be, by law, equally easy and straightforward as through their own stores (i.e., no security warnings or other jank). The law should make illegal restricting developers to any specific payment processor(s), too.

Not the same kind of regulation you describe, but still one that may solve a big chunk of your issues was covered here roughly a month ago[1], In which the EU plans to make those kinds of platforms more transparent, especially concerning bans and sudden refusals. But I see how that may not go far enough.

[1] https://news.ycombinator.com/item?id=23841409