I wonder if most of the pain is actually caused by Rust async being an MVP, so things like async trait functions (which would be very nice) don't exist... yet.

I don't know if anybody has shown that they can't ever exist, it's just that they weren't considered necessary to get the initial async features out of the door. Rather like how you can't use impl Trait in trait method signatures either (there's definitely some generics-implications-complexity going on with that one).