same issue here - did you manage to find the solution?
I have had that rebooting loop with 5 plus twice and both times was able to manage it.
First time the loop was caused by unsuccessful firmware update through the Android connect app. I was able to manage it by connecting to the desktop Garmin Express App and multiple factory reset (google it). On the 5th or 6th reset my 5 plus finally was connected to Garmin Express and re-updated firmware (by itself, I didn't do anything specific for that but reset). Since then It worked well.
Second time it was caused by bad music file (it was really wrong). I was able to manage it by deleting bad file in File Explorer in Windows while connected through USB. Note that "bad" file generates an error in any Windows any player but causes reboot for Fenix.
My watch is doing the same bit it doesnot want to switch on at all
Ring product support about the blue triangle screen master reset procedure......
Its something like this...