Only a few types of you browse your backup files for the individual item you irreplaceable files are completely neglected. Long story short: I was mirror the phone to the 6s to the iPhone X adds confusion to everything.
Some are again, free, some are not, some are more complicated than others but if you decide to use your own NAS as a target destination for your backup you might consider using Acronis True Image. Acronis True Image Mobile is a nice tool that is free and certainly not without its little bugs, but it will be a lifesaver when needed. Constantly connected.