That Linux phone is called Android. It runs plenty fine enough even without GApps (or with shims like microg), and the sheer amount of engineering needed to make baseline linux even usable as a phone system is over a dozen years away.

Android with binder is a strictly superior architecture that anything else that has come for strict isolation. As a bonus, it's battle tested, and latest Android phones just... run linux. You can have a shell and GTK if you so desire.