Did full disk clone w/a few diff tools. Problem seems to be:
(orig setup)
C: on SSD1
UEFI + D: on HDD1
(clone's setup)
C: on new nvme
UEFI + D: on HDD1 (unchanged)
* Will not boot
Tried adding UEFI partition to new nvme, unplugged SSD1 and HDD1. Nope.
Tried adding UEFI partition to SSD1, then cloning to nvme. Nope.
Basically... just gonna nuke it from orbit and install a fresh Windows 10 on the new nvme with no other drives connected. Then I'll be guaranteed to have a self-contained UEFI + C: universe on the nvme. Then will back HDD1 as a data drive and wipe its UEFI partition.
ps - I hate that I know any of this. I didn't want to learn about any of this.