@Butters, Cnfhbr
Thank you for the help. While waiting any response from Garmin I'm looking other ways of troubleshooting.

Fenix 3 HR firmware is distributed in .rgn files - [Only registered and activated users can see links. ]
It can be opened easily using RGN tool, and contains single firmware section:
Spoiler: pic 1
[Only registered and activated users can see links. ]

Opposite to new gcd firmwares which are split into 2 sections (and are encrypted according to Cnfhbr):
Spoiler: pic 2
[Only registered and activated users can see links. ]

I might override 2413/500 (hwid/sw_version) into 3288/1660 and flush it into Fenix 6S Pro using Updater.

What do you think about this way? Goal is to make it visible as storage device, replace corrupted .gcd and reboot.
Does firmware strictly depends on hardware for pre-boot phase?