I think this might be leading me down a road that could actually work a lot better than what I currently have.
Say you have a threadServer (that is, a long-living thread that exists besides your main thread that you can send messages to and receive messages from). You could also have a third thread, a "helper" thread, whose sole task it is to read messages from the channel for the threadServer blockingly and transform them into async work that it puts on the async-work-queue of the threadServer. That way, when no message is in the channel, the "helper" thread blocks on read, spinning it down to a more energy efficient state. Meanwhile the "threadServer" itself only does one thing as well: Read from the async-work-queue and work through it. That way message processing happens and if that spawns more async events (because async-IO happens or sth) then that's not a problem since that thread **only** processes async-events.
