I think the power button shutoff can happen like that sometimes. I've seen that before, but not 100% of the cause. May want to run onboard video, and uninstall the GPU drivers for the card. It's likely on Ultra-fast boot, and trying to load drivers right after the splash screen. Worth a shot at least, but seem odd that BIOS entry won't work. If you can enter with onboard graphics, I'd look for a CSM setting and see if it's enabled and turn off Ultra fast boot, at least temporarily, perhaps even secure boot.