After using 2.9 and it failed at flashing firmware 0/6 I managed to get my system back by using the downgrade method also someone had provided me with the 6.1 exFAT fw mod that you need to create in step 4. The file I also had been given did not have the USER/SAFE folders and still worked for the downgrade with skipping them.
I had no NAND backup but I do now after I got it to boot on 9.0.
Just make sure to upgrade to 9.0 using 2.8 before you put 2.9 back on.
Im pretty much stuck on step 4. i have tried so many keys but none work. Can you provide me the 6.1 exfat fw mod? Ive been going crazy for the last 3 days.