As much as I don't like to, I do use Google things on phone, and I've really only ever used secondary profiles to use shady apps with far too broad permissions, such as the kind required to update firmware or get data from wireless/Bluetooth devices.

If you do want a smart phone, but don't want rubbish on it, Graphene is very bare-bones by default. F-Droid and other FOSS-friendly app stores can fill in a lot of gaps, and respect your privacy.

Some apps will simply not run without Play Services, but if you had to use such an app, I believe you could do it in a second profile and have Play Services disabled in your main. Regular apps installed for secondary profiles show as "Not installed for this user" in my main, but Play Services is a bit special.

> I may very well be assuming things about G-OS that it doesn't yet have (or may never have).

No offence but... next time maybe make it clear that you are just assuming and don't have any experience with the thing you are describing?

> My mental picture would have been it having partitioned storage (to reduce chance of accidental over writes) filled with "random seeming 'noise'" that held hidden account specific data only accessible with a user provided key.

That would be a normal account on an encrypted phone. Nothing special about that.