Is it possible to temporarily skip media restore in WhatsApp?
The important thing is that you restore the backup on the iPhone 6. Once restored, it disables the backup of that iPhone 6 so that it does not replace the original copy of the iPhone X. Then when you have your iPhone X again, you will be able to restore the copy original.

On the other hand I recommend that you make a backup in iCloud of all the content and a backup in iTunes.