Isn't the use or non-use of async/await a bit orthogonal to the rest of this?

I'm not an expert in this area, but wouldn't it be just as lightweight to have your async workers pushing onto a queue, and then have your async writer only wake up when the queue is at a certain level to create the batched write? Either way, you won't be paying the OS context switching costs associated with blocking a write thread, which I think is most of what you're trying to get out of here.