Music sync on watch is always slow regardless. I am not sure watchOS 6 will improve that but I doubt it. Apple clearly wants to force user to NOT use iTunes at some point. But we will see.
For now, try and see if watch can connect to the same wifi network as iPhone, as it might transfer files using faster wifi instead of slower bluetooth. My test in watchOS 4 shows with Wifi, 8GB of data can be transferred in an hour or 1.5 hours, whereas using BT, it can easily take 4 hours if not even longer.