I don't think c# is the right language for an activity pub server. There's to much "fuzzy" - properties that could be a list or a string, or one of a bunch of broadly similar types that aren't really sub-types.

If I had a JavaScript engine I was happy with, that could work, but I don't.

F# feels like it's probably a good fit, and this is probably the right size of project to start with, but I'm not sure. F# feels like a "write only" language (like Scala) in that it's very dense and the compiler picks up much of the slack, so getting a hook into understanding code is tickets.

I'm tempted by C. I can write a basic JSON parser (which knows that it's not going to have to deal with extreme cases), but even so it's the same old C problem (that there's just so much more to type)