Definitely in favour of a smaller class library. JITLib on particular could be moved out. There is also a bunch of old unused code floating around.
What annoys me about quarks is that they don’t have descriptions, and when you go to the GitHub page, most people’s quarks don’t have a readme either. Some stricter requirements might be needed before quarks are added. I’d be in favour of kicking all those that don’t meet the requirements out (after a request to update the description/documentation). There could be some really good code in there, but personally I’m never going to install something with an indecipherable name and no complete description, doubt I’m alone.
Regarding synchronization stuff, supercollider deals with asynchronous relationships in almost every action the user makes because of the server. For that reason, I think this should be baked into the core library in a far more substantial way, and all classes should be more aware that something might wait. One example, if you wait in the .asString method you break the interpreters ability to post window, which isn’t It’s an unreasonable thing for the user to want to do.