Windows 10 inaccessible boot device after transfer to SSD

OK, I gave this whole thing a second go and - success! I'm not sure what I did wrong the first time but here's how I did it the second time:

1. Created only one big partition on the new SSD, with MBR (not GPT) scheme
2. I downloaded fastcopy to my winpe and used that instead of xcopy (mainly because of junctions): https://ipmsg.org/tools/fastcopy.html.en
3. Removed BCD on the new disk, and used bcdboot instead of bcdedit to create a new boot config with the correct drives
4. Finally, and this was one of the crucial steps, from winpe, I changed the drive assignments to make sure the new SSD is C: (via regediting HKLM\System\MountedDevices) in the new windows dir.

Then reboot and ta-dah! Running from SSD.