Q: Incomplete synch 2 iphone

After spending several hours organizing my music library on my Mac and populating it with specific playlists to be copied to my iPhone,I followed the instructions from https://support.apple.com/en-us/HT201593 to the letter and then walked away (as the files totaled around 20GB).  When I came back some time later both my Mac had reverted to screen saver and my iPhone's screen was off. I activated the displays and logged in to both the Mac and iPhone and could see no indication at all that the synch was still in progress, so I disconnected the phone from the Mac and proceeded to browse through the playlists on the phone and on the Mac.

Not only were a number of the playlists I had checked off NOT copied to the iPhone, but they were also no longer in iTunes on my Mac.  Luckily all the songs were still in the library, but now I need to spend even more hours re-organizing my extensive music library in the hope I will be able to get this to work right the second time.

What do I need to do to make sure that EVERY playlist that I check gets copied from the Mac to the iPhone and how do I prevent anything from being deleted from iTunes on my Mac in the process?

Editorial comments about this boondoggle would be tempting to make, but I will restrain myself in the interest of keeping with the guidelines for this community.