I agree that we only have the choice between the devil and the deep blue sea concerning mobile OS.

But why is iMessage so great (asking as a non-apple user)? Is it even e2e?

Great alternatives which are not locked into a specific OS are Signal, Threema, and Matrix.

Specifically, Matrix -- which is also a protocol -- looks quite interesting to me. Similar to email, you can choose your client software. You are also free to set up your own matrix server but you don't have to.