Ah yes, in that third picture, it says Boot Menu (F8), that should list all your bootable, connected drives.
in all my years I have never flashed the BIOS back a version, always forward, but you will definitely not lose and data on your drives. the absolute worse would be the BIOS gets bricked and the PC won't start, but the drives data will be unaffected and can be connected to another PC for retrieval.
but again, in all my years, I've never seen a bricked BIOS from flashing, although you do hear about them.
the reinstall of Win10 from a USB stick would probably not help in this instance although i certainly couldn't rule it out, and would try it if nothing else is helping, if for no other reason than I'd have a clean install of Windows.
if the NVMe drive isn't being detected as a boot drive after the BIOS update and it was beforehand, than logic would dictate that reflashing back to the original firmware would fix that.
BUT... part of the re-installation process of Win10 would reconfigure all the partitions on the NVMe and that may get you to the same end result. if you take that path, doing the re-install process, Windows gives the option of deleting all partitions before reloading, I'd be doing that.
so, I'd reflash first as it should only take about 5 minutes.
and I'd only reload Win10 if that didn't help or if I simply wanted to refresh my OS.