Finally made some time to revisit this today.
I shutdown, disconnected all SATA drives, moved the NVMe from PCIe x16 slot (as x16) to PCIe x16 slot (as x4), restarted, went into BIOS, reset to factory defaults, restarted, went into BIOS, changed Legacy to UEFI, apply, restarted, booted to USB as noted above, and performed OS setup in +/- 4 mins.
After it prompted me to restart, I did so, went into one-time boot options to make sure that 'Windows boot manager' showed up (it did now), and booted to that.
Screen goes black, stays black for a long while, hit ESC to see what would happen, and entered a UNetbootin 'Automatic boot in 10 seconds' countdown loop.
Googled that and found a thread that describes this as being a UNetbootin issue - something I only used at Samsung's direction (from their firmware update doc), mind you.
As a result, I'm bypassing UNetbootin altogether, and currently redoing the bootable USB using Rufus... Will attempt that as soon as I can, otherwise I'll go old-school DISKPART procedure which has never failed me (just takes a while, by comparison). Haha