It is indeed not as straight forward. It's actually a massive pain going from an engine module to an extension, as the API exposed is what you get in GDScript, not what you get in C++ when writting an engine module. It's similar enough, but requires tons of specializing if you want to keep both a module and extension build. We'll keep providing both.
The web and C# story is still pretty much up in the air. However, W4 Games seem to be working on (or at least looking into?) that. We've been exchanging emails about spine-godot as an extension for a few months now, so I keep reporting the pain points. Whether that means Godot itself, which isn't owned or managed by W4 Games, will fix this up, I don't know.
I actually kept asking for a guide or build system template to cover all bases. But nothing like that exists at the moment.
