Frankly I had thought you'd only have a small chance of fixing it with a new battery because a battery charge level problem usually results in spontaneous shutoff in most devices rather than reboots or bootloops.

Because you have a bootloop, which if not attributable to a faulty file during the loading process, it's probably caused by a physical hardware failure. It's usually the flash memory chip failing before dying. So unfortunately, I would say that if you're absolutely sure you didn't re-use any old files from a backup that an impending total flash failure is most likely.

You could try booting it with only Garmin folder, etc., as explained in (iii) & (iv) of Post #9 to see what it does then. However, first re-do the format, ensuring you have the correct settings as shown behind the "program screen" Spoiler in the RMPrepUSB Thread: [Only registered and activated users can see links. ].