Ok, last gasp attempt and if this fails it's game-over from me. The problem is if SD flash doesn't work that indicates that fw_all isn't even starting, therefore likely region 14 is shot. That's where the main 'system software' (firmware) is stored.

So try this:
  • Remove the Ldr.bin file from 0927 0972 folder and ensure that the file update.txt is the only content there.
  • Open Ldr.bin/boot.bin in a Hex editor program (such as XVI32 for example) and search for the first instance of "0C 80 A0 E1" (4 bytes aligned) and change it to "0C 80 A0 E3" then save the changed file (Read more [Only registered and activated users can see links. ] if you wish).
  • Open the changed BIN file in RGN_Tool and save it as 09720100630.rgn then open it in Updater.exe.
  • Ensuring the device is fully OFF, insert the microSD with Garmin\Updater\09270972\update.txt.
  • Start the device in preboot and attempt to flash the RGN file.

If it won't flash with modified boot.bin as RGN then it's almost certainly flash damage. It is rare but it happens occasionally and there's no software solution, the only way is to replace the flash chip (it's BGA so that's not an easy fix) and then re-program it (again, not an easy thing to do). Read here: [Only registered and activated users can see links. ].

Please let's know the result, likely that's only morbid curiosity to be honest.
Typos fixed as requested by poster.