It seems you've done everything right. First please confirm that the unit has usual US/EU hwid of 0754, it will be shown in G7ToWin without the leading zero as 'ID 754'. Once confirmed please read the rest below but if it's other than 754 post to advise.
Perhaps the flash is damaged but if you're brave enough there is one more thing you could try, it worked for me with a stubbornly bricked 1350 recently. I'll give more details after we establish that it can accept commands from the SD so firstly you need to check if the unit can still do that. Try to dump both region 41 and region 48. Region 41 is of course the essential 'nonvol' and 48 contains the file system usually visible in Mass Storage Mode. Copy this text command into a new .txt file named Update.txt using NotePad:
On a CLEAN SD card of minimum 8GB size, create a root folder Garmin. In that create another named Updater and in that one create another named 0754. In that 0754 folder place the Update.txt file along with a copy of boot.bin from the firmware, but name it Ldr.bin [note that V4.70 was withdrawn by Garmin so use V4.60, it may well be that V4.70 is the reason it's bricked]. So you'll have this - SD:\Garmin\Updater\0754 with the two files in there. Ensure the unit is off, if it's not then hold the power slide to the left for up to 30 seconds to force it off. Insert the SD and turn it on, preferably by connecting it to a power source if the battery is weak or not fully charged. If 'Loader' shows on the screen it's copying to the card, let it finish and try to reboot but it won't be able to complete that command and will go back to a bootloop. Force it off and remove the card then check what's on it. Hopefully you will have 41_backup.bin and 48_backup.bin in the root. If not, check the 0754 folder to see if 'update.log' has been created, if so open it in a text reader and report its content. If it doesn't work, try modifying the boot.bin and use this method from Post #38 to run the command by flashing the modded bin file as an RGN in preboot: [Only registered and activated users can see links. ]. Be sure to remove everything else from the 0754 folder except the Update.txt file in that case.