If you need a locked down phone that passes remote attestation to authenticate yourself to a remote service, then whatever you use to access the service UI doesn't really matter: the only device that's necessary to have to use the service is the one you don't fully control, and which gets to control your patterns of use.
An intuition pump I like: imagine you want to put a widget on your desktop that always shows you the current balance of your bank account. You want it to just work ~forever after initial authentication (or at least a couple weeks between any reauth), and otherwise not require any manual interaction. See how hard it is (if it's even possible), and you'll know how badly you're being disempowered already.
https://www.paloaltonetworks.com/cyberpedia/what-is-the-purd...