I don't know if this will help with your issue or not, but it is trick which works with the "Watch Player" podcasts app, so might be worth a shot?
Before trying to transfer anything to the Apple Watch turn off the Bluetooth via the "settings" app on the iPhone, NOT the slide-up "Control Center" as that doesn't fully disable Bluetooth, which is a whole other issue! No need to change anything on the Apple Watch.
Make sure Wi-Fi connectivity is still enabled for both iPhone and Apple Watch, and that they are both on the same Wi-Fi network, then sync the content. This forces the watch to use the much quicker Wi-Fi connectivity rather than the slow as **** Bluetooth connection.
Once finished remember to turn your Bluetooth back on!
I have found that in the case of the Watch Player app, that hour long podcasts (around 75mb) transfer in a few seconds, rather than what seemed like hours using the normal method.
As I say it might not be the answer to syncing songs, but it could be worth a try. Hopefully Apple will stop neglecting such basic functionality with the watch and address this crazy situation with a software update.