iMac 2013 (late) as external monitor for PC
There's nothing you can do on the PC side.

In order to use the iMac as a monitor, it appears that a thunderbolt
display output at the source is required

This is a misunderstanding of how this works.  There's no such thing as &quot;thunderbolt display output.&quot;  Thunderbolt is a multiplexing of a number of signals:

PCIe
USB
DisplayPort
Power (Thunderbolt 3)

If it was just the &quot;display&quot; portion of Thunderbolt, it would work with your mDP to mDP cable.  You need Thunderbolt connections end-to-end.