C:\Engineering\Development\Qt\Tools\QtCreator\bin\jom -j 8
I wanted to keep the versions for the standard and static versions consistent, but its not like it would really matter since its just a small revision difference. So, I attempted to build 5.12.3 this time and while I was at it I grabbed the dynamically linked version of 5.12.3 from the maintenance tool as well.
This time I ran the Visual Studio Native Tools command prompt as an administrator since I figured that had a shot at mattering even though I didn't see it mentioned anywhere and wasn't doing this is a UAC protected directory.
And... it built completely with no issues and I was able to produced a statically linked version of the app I'm working on like I wanted to. I don't know if there was a bug in the 5.12.3 source, if it actually was running the prompt as administrator, if there was just a simultaneous access issue with some of the files/tools on the last run, or if it was just a fluke, but everything works fine now.
The build ended on the same two lines but succeeded in copying qgltf.pdb instead of not finding it and the process took much longer than the previous attempt, so I'm guessing that there were errors earlier in the process that didn't stop the build script so I didn't catch them.