Well, this was unexpected
I thought that your flash chip is broken and it can't flash other firmwares except the one that was flashed before it broke (some flash blocks are "frozen" with some bytes forever; writing differing bytes causes an error). I've seen this before. And your situation looks very similar. So I was going to confirm that.
But now it looks like something different. boot.bin can't flash firmwares, but bootloader can do it. Wow.

Now you can try following the GarminCure3 tutorial and format the drive with RMPREPUSB. But make a backup first.