Welp, the issue's been fixed.
Idiot me just realized that after the BIOS update, the BIOS got reset to defaults and the default CSM state is DISABLED. Since my SSD was set up in legacy (MBR) mode, the BIOS could not recognise the boot partition and boot Windows from it.
Enabled CSM and the system booted fine.
Have now converted the SSD from MBR to GPT using Windows' own non-destructive
mbr2gpt (available from build 1703) and disabled CSM in BIOS and am back up and running.
Excellent link for conversion with the mbr2gpt tool here:
https://www.windowscentral.com/how-convert-mbr-disk-gpt-move-bios-uefi-windows-10"]*How to convert a...The process is very quick and painless.