I've tried using the Rock Band Blitz game as an entry point for the exploit and I'm on my 15th or so try, each lasting from 5-15 minutes (mostly 5-8 minutes). Still no luck.
I've run into the issue where I pressed the A button too soon as well, but that one's on me. 30% success rate isn't really going well for me. What could be the reason? I followed MrMario's USB setup, I highly doubt that I've made an error there. The USB drive should be fine, I've tested it with the h2testw before attempting this exploit. I get to the part when the lights start alternating diagonally and after a while the screen freezes and sometimes it it has 3/4 of the ring activated, the other times it's one of the diagonal patterns. Whenever that happens, I reboot and try again. In most of the attempts, my USB drive has been connected to the one of the ports in the back, while in the remaining few it was connected to the one of the front USB ports. As far as I can tell, no change. My controller is not connected to the console via USB just in case that might cause an interference with the exploit and the USB drive is the only USB device connected to the console.
Any suggestion what to try next?
EDIT: I checked the LED patterns
here and I'm not sure how these translate to the Slim consoles, so once it froze for me but the ring had 3/4 activated I waited for a while and, sadly, nothing happened.