With the vendor/system split introduced by project Treble in Android, it should be easier than ever to build your own system against a rich set of hardware abstractions, that work on a wide range of devices. Assuming you are ok with still running a very thick slice of the stack as proprietary vendor image.
Yes you can run a GSI (and project Droidian does that) but then you're dependent on a downstream kernel and Android-ish early boot environment, that will likely lead to pointless incompatibilities compared to a fully-upstreamed approach.
The biggest hurdle to getting AOSP-kernel features into the upstream kernel is not clang or Rust, it's cleaning up hacked-together kernel code in a way that makes it long-term acceptable to upstream maintainers. (And getting rid of userspace blobs for things like graphics.) Always has been for as long as AOSP was a thing.
