iPhone 7 says not enough space to sync music but there is
I have an iPhone 7 (no SIM card) and just updated the software to 15.8.3.
My iTunes is on PC, version is 12.13.3.2.
There is 27GB of free space on the iPhone; my entire music library is 20GB.
When I try to sync the library, I get a message saying it can't be synced because there is not enough space to hold all the items. At first, it said I needed an additional 250MB, tried again and it said 500MB, and now seems to be stable at about 900MB. Tried rebooting and updating all devices involved, still the same issue.
I have very few apps (all updated) on the iPhone, and no other media whatsoever.
Does anyone know why this is happening and how to correct it?
Thank you for your time and attention.
iPhone 7, iOS 15