Hey everyone, I have also been having this problem and have finally found a solution that is not quite what anyone else has posted yet.
I actually haven't seen the "Syncing Music to iphone" since the new ios update so im quite happy..
Here are the steps that I took to solve:
1. Turned off auto lock and password on iphone
2. After connecting iphone on the summary page the only three boxes I have checked are Automatically sync when iphone is connected, sync only checked songs and videos, manually manage music and videos
3. Now, click the On this iPhone tab. In the bottom right there are "setting" and "Autofill". Under "Settings" select replace all items when Autofilling. I also arbitrarily selected "reserve 1.12GB for disk use".
5. Then I clicked "Autofill" and selected Replace Songs.
6. Lastly I pressed "Sync" while I was still on the "On This iPhone" page.
7. I was actually about to leave the computer and then I saw something magical..."Syncing Music".....Holy Sh...!!!
Notes:On the iPhone: WiFi, Bluetooth, Cellular, personal Hotspot and VPN are all disabled. I use PC. I have about 5000 songs. I do not use Match.
Before today I was also having a disc space issue with a large block of "other". I deleted the music data from my iphone and that fixed that problem, however, the phone was still not syncing the majority (4800 of 5000) songs.
I do not pay for icloud storage
*I also do not have the "Sync Music" box checked under the Music tab, however, simply doing this did not solve the problem