Restore backup from iPhone SE (iOS 13.4.1) to iPhone 6 (iOS 12.4.6)
You can’t get Apple to restore a newer OS to an older one. This is by design. Short of a jailbreak you will need a device that runs iOS 13.4.1 or newer to restore the data and then examine / export / sync or write by hand or manually any data for RE-entry in the 12.4.6 device. 

iCloud sync is a very good way to go if you can borrow or rent a device assuming you don’t have both in hand and working.