There’s GrapheneOS that I think would try to address this problem — secure, proper architecture, compatible with some major app stack (e.g. Android apps). It’s AOSP-based, but they’re already thinking ahead up to a point where they would be forced to fork it and even work with OEMs to create their own phone hardware for it. There are a couple of threads on their Mastodon.

I don’t know how much they would be able to achieve, but I would pay for such system.