Transfer music to iPhone without Sync and Delete

This is a user to user support forum. I can tell you how things are, not how you or I would wish them to be. Back in the past Apple made it such that copying content from a device back to a computer was hard. Presumably this helped get content providers who were fed up with home taping on board with the digital music revolution. When iPhone was introduced they added (for reasons that baffle me) another limitation that means even a manually managed iPhone can only be managed with a single library/computer. If you backup and migrate your library properly then all will be fine, but if something changes, and the library on the computer appears to be novel from the perspective of the iPhone, then you will get an erase and sync warning. Pay heed. It means what it says. There are many many threads on these forums from people who went ahead and tried to sync a full device with an empty library and ended up with nothing. Take my advice, reclaim the content from your device and add it to your library. Deduplicate as necessary, then make a selection of what you want to copy to the iPhone. You can try third party software, paid or free, or a short subscription to Apple Music, but one way or another you need the music on your computer, not just the iPhone, or you'll end up having to go back to the original sources, or losing things.

tt2

See Recover your iTunes library from your iPod or iOS device - Apple Community. Once you have everything in your local library, and ideally backed up, it should be safe to erase the iPhone and reload it from the library.

tt2