I think I understand the issues well enough.
You can switch a device from automatic syncing to manual management without iTunes wanting to remove the content that is already on the device, unless you are connected to the library that loaded up the media to it.
With Apple Music/iCloud Music Library enabled on the device you manage what is available to the device by adding content to your iCloud Music Library on your computer. On the device you can then choose to stream content or download it for offline use.
You can backup your device and transfer media purchases into your local library without affecting the content that is on the device. You should do this now whatever else you choose to do..
You may also be able to extract unprotected media that was previously loaded up on the device. You should probably try this method now, particularly if you do decide to iTunes erase & sync. Erase and sync should affect the content of your iCloud Music Library, but anything you download from it that wasn't an iTunes Store purchase will likely have DRM attached. This locks you in to an ongoing subscription unless you want to throw away your non store purchases.
iLounge Article |
Copying Content from your iPod to your Computer - The Definitive Guide. Page two of this guide shows how to extract all your media using Windows Explorer or Finder. It is mainly aimed at older devices that can be accessed via disk mode, however free tools such as iFunBox or MediaMonkey which give access to the iOS file system may also allow you to use this method to extract media from an iOS device. Windows users would be able to make use of the two scripts above to recover playlists and stats. Recommended for retrieving everything from the device in one go. |
For these video files that you want to add you may be able to use an application like CineXPlayer or VLC to load these up to your device independently of the iTunes syncing mechanism. That might give you time to consider the options. Transfer of files using the iTunes > Device > Apps > File Sharing area shouldn't affect media on the device.
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