So, while investigating the blue screen; I found dcrypt.sys (DiskCryptor's driver) was listed in DumpFilters at Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\CrashControl\
Since I no longer have any encrypted drives, I decided to uninstall DiskCryptor to see if that could be interfering. The uninstaller failed to remove the dcrypt.sys driver, so I manually found it in C:\Windows\System32\drivers and failed to delete it as it was in use by a process (Explorer didn't want to tell me which process unfortunately), so I grabbed IOBit Unlocker and force removed it.
I downloaded the update directly from
Download Windows 10 this time instead of going via the Windows Update screen. (which I had tried previously without success)
It failed the first couple of times as it claimed a setup was in progress, I stopped the Windows Update and Windows Installer services, ran
msiexec /unreg and
msiexec /regserver in an elevated command prompt and killed off any processes which referenced setup.
The Windows 10 update assistant proceeded, rebooted; first reboot at 75% and then it rebooted and completed the setup.
I'm now on 1803. It's a miracle.
I really appreciate the help!