I'm so sorry. I wish I'd seen this thread before you went too far and ended up with non-working backups.
For the future - know that for the past few years you could have been importing your photos into Mac Photos, or into a pictures folder on a PC, before you wiped your phone. Importing into Dropbox first is much slower.
You could set up 2 factor authentication and upgrade your iCloud account to allow you to start storing iMessages in the cloud, along with all of your photos, and to have a second backup in the iCloud. You can keep original photos on your computer and the iCloud at the same time, and the iCloud backup is like a "backup of your iTunes backup". All my computers have more than 1 backup as well.
I would have also tried the REIboot app to reinstall the same current version of iOS on both phones without losing any data, while restoring in iTunes wipes the data on the phone. When my wife's iPhone 7 went into a continuous boot loop and her most recent backup was 4 weeks old, I was able to save everything with REIboot (it had stopped backing up to the iCloud 4 weeks earlier due to a corrupted iOS, and we didn't become aware of the problem right away).
You may have to wait for official iOS 12.1 to come out before being able to restore the 12.1 beta backup to any phone. I would expect that the "older" backup might go back onto the phone once the iOS is newer than the backup.