Device to device migration

It is not about the age of the phone, it is about the iOS. You cannot restore a phone with an older iOS (your XS) from a backup made with a newer iOS (the 12). You can restore a phone with a newer iOS with a backup from an older iOS, just not the other way around. Sorry.

No, you cannot move content from a newer iOS to an older iOS. That includes backups and Quick start. Unfortunately, the update that was provided for the iPhone 12 series has caused this. While you could never do it in the past, trying to move from an iPhone 12 series to even a slightly older device is impossible to use the backup.

Sorry. You can try contacting Apple Support to ask them, but I don't believe there is a way around it. Just make sure you have a good backup of your iPhone 12 before you send it in.