Very interesting writeup! The actor system you describe is exactly the kind of threading story which would really help Nim. A programming language which in 2023 doesn't have anything easier than manual locking and such isn't exactly a great look. Unfortunately it sounds like Nim currently fights your attempts at getting this done fairly hard..
But your approach is still very interesting. If Nim could just be a bit friendlier about handing of one tree of ref objects to another then that would be a very nice way of dealing with threading.
