A device showing 'System Software Missing' defaults to preboot mode (aka Garmin mode). Garmin calls their firmware 'system software' which it is of course. The message is usually indicative of region 14 (0x0E), which holds the main system software, either being empty or corrupt. That can result from an incomplete flash or incorrect data being flashed (a wrong firmware). It's then often easily fixed by flashing original firmware provided a wrong boot.bin/Ldr.bin hasn't also been used previously. The other reason that the message can display is because of physical damage to the flash and that's not correctable by software means. In your case it seems the device may not be recoverable. Your report implies that it was working when you stored it and accordingly it's assumable this is a spontaneous result of the initial boot after recharging. If you didn't update it or flash anything to it before storing it then the only reasonable assumption is that it's hard-bricked thru flash damage. That's supported by the fact that G7ToWin fails to recognize it and Updater reports it's UID as "0" indicating damage extending to other parts of the flash, i.e. other regions as well as 14. This is beyond my expertise to advise you further, sorry, you've already tried everything i would have suggested. Perhaps someone else has another suggestion or more ideas to try.