The key question for me is whether this "advanced flow" will allow the practical use of entirely separate app stores (like F-Droid) or if they're going to throw up tons of barriers for every individual app install.
If I were designing the advanced flow, I'd require the decision to be made at phone setup time. Changing your mind later requires a factory reset. Real sideloaders (F-Droid users, etc.) know at setup time that that's how they'll be using their phone, so it works for them. But ordinary users who are targets for sideloading malware will become a lot less attractive if attackers must convince them to wipe their phone to…
But wiping your phone isn't "a little pain"