> Do you have any good resources on OCaml5 effect handlers I can follow?

I encourage you to have a look at the published paper, it's an easy read and covers the motivation and examples:

https://arxiv.org/abs/2104.00250

Here's an excellent talk that walks through modifying a non-trivial code base for concurrency using effect handlers:

https://youtu.be/k3oQwpyXmpo

> It does feel to me like error's and async's both should remain orthogonal

They aren't completely orthogonal though, as both are effects. Haskell and Rust model both with Monads. OCaml 5 can model both with effect handlers. It is desirable to track the difference in types, this is something I hope the OCaml folks will add in the future.