Reboot loop after Bios update

Hi all,

Seem to have an issue where my PC is just reboot looping after trying to update the bios. For reference this is an Intel 13600kf in an Asrock Z790m PG Lightning D4 board. It came with bios 5.03 and I was trying to update to 9.04. System was working fine and stable prior to bios update (If you ignore the very odd graphics glitch related to Arc graphics cards that a quick vid restart would cure)

Followed instructions, downloaded bios, unzipped and put on fat32 USB stick. Into bios and run update util, bios found and update starts. All seemed to run fine, got to 100% and restarted. Then the reboot loop began. Left it run for about 5min with it getting to Asrock logo for barely a second, then reboot, rinse and repeat ad infinitum. Spamming Del or F2 doesn't do anything.

So far I've tried resetting Cmos via jumper (5-10 secs) and battery removal (up to 10 mins) with no joy. The bootup and post status led light up goes as follows:

Turn power on: Dram, Vga, boot lit, cpu out
Dram and boot go out, vga stays on
Boot comes back on, vga stays on
Asrock logo appears on screen for less than second
System reboots

I've removed ram down to 1 x 16gb stick, usually run 4

Changed gpu out for another (Using Arc A770, swapped for known to work 1050ti)

Removed 2nd nvme drive, ssd on sata and all USB devices apart from mouse and keyboard.

Still same happens, so feeling like bricked the board with failed bios update. Board itself is only 6 months old, but not sure if I can rma due to failing a bios update.

Any ideas? Fairly PC literate as have been building own PCs since my 1st athlon 850! Kinda annoying as use it for work, looks like I'll have to resurrect my old Xeon E3-1231 til I can get it sorted.

Thanks in advance

Jon