What you want isn't a "linux phone", what you want is a gun to put to the developers' heads so you can scream "let me tamper with your shit".

If you just want "a phone OS", AOSP is still there and worth forking. But you don't want a phone OS, you want apps. And nobody is going to write apps for an AOSP fork (see also: Fire Phone). Actually, nobody is going to write apps for anything other than Android and iOS, just in general (see also: Windows 10 Mobile). App development for two phone platforms is already enough of a pain in the ass. Furthermore, Google will absolutely be anticompetitive and de-Google your phone OS whether you want it or not.

But more importantly, if you do manage to create a third platform that people actually use, you are going to immediately be inundated for requests to lock down the phones in exactly the ways you object to, because a certain subset of app developers want or need that kind of DRM. And you're not going to get those apps without a DRM story that matches Google and Apple's.

Streaming apps want encryption to the monitor.

Games want a kernel the user can't modify.

Banks want your phone to be a credit card you can't do fraud with.

Hell, when macOS got support for native iOS apps, they specifically designed it so that iOS App Store apps won't run if you modified the OS in any way. And even then, a lot of iOS app developers specifically blocked macOS usage. The phone vendors aren't selling an OS, they're selling DRM.

I wonder what an alternate history where Google didn't implement remote attestation and actively made it difficult for apps to snoop on user modifications would look like.

I suspect the Fire Phone is where things changed. If Amazon had iterated on it and done a good job, it might have been a competitive threat to Google's revenue model for Android; that's less likely if it won't run games and banking apps.