I wrote a similar solution based on reflection for a game server I wrote in Go. It's probably too early to be worrying this about but reflection is apparently slow. I also found it difficult to write reflection code that was robust.

The problem I'm stuck on now is that since my "Unmarshal" function returns interface{}, I have to type assert for every type of packet in the protocol. There's probably thousands in total. I need some magic that can automatically Unmarshal to a concrete type and pass it to an appropriate handler. gRPC's codegen does this for you. Still working out how best to do this in the constraints of Go.

You can type-cast from interface{} into any other interface, so something like this might work: https://go.dev/play/p/bhrXr8-V7Gc

I do find the documentation/packaging rather unclear. For example the website mentions an IDE, but when you download it, there is no IDE in the downloaded package(thats what I remember). Exactly details I'm not remembering, but the Java version documentation was rather unclear, had to work with a bit before understanding how it worked.

Actually could not figure out how to parse a binary file/stream containing heterogeneous binary structures from reading the docs. I suppose it is possible?

It's some time ago I had to parse a binary file, but your go struct annotations look great. Adding streaming (un)marshalling could be a killer feature for some.