Recreate iTunes library on laptop and sync with iPhone

To rebuild your iTunes library: create a new empty iTunes library, point it to your old music folders, import all audio files, and then re-enable syncing to your iPhone. If an old XML playlist file exists, you can restore playlists. After import, you’ll have a clean, modern iTunes library that syncs properly with your iPhone again.

See Recover your iTunes library from your iPod or iOS device - Apple Community. The easiest method may be a temporary subscription to the Apple Music service, merging the music on the device into an iCloud Music Library which you then download to a computer and backup for security.

Once you have the library of items on the device on your computer as well (and ideally backed up) you can afford to proceed when you get the erase and sync warning.

Using multiple libraries is best avoided if you can. Apple devices like to take content from one library at a time. It should be possible to add all the content you have into one library. See Duplicate songs in iTunes - Apple Community to clean up any duplicates you might generate while merging collections of content.

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