Sorry, I hadn't come back to trying this for a while.
I was able to get past this by dropping -optimize-size, but ran into other strange issues, primarily where a purely debug build had a bunch of errors and a mixed config build would not create the debug lib files (likely due to the same cause, just the errors didn't show or halt the build for some reason) even though the process appeared to complete successfully.
Not having much luck with Qt6 it seems, which is a shame since I'd really like to make use of the upstream changes I submitted for it.
Luckily at this point MSVCs internal CMake version has been updated to one that is compatible with Qt6, so the manual replacement is no longer needed.
I'm currently trying to build 6.1 and was able to get past the configure step, though I had to disable tests (which this time I wanted to confirm it built correctly) due to QTBUG-88829, so I'll see if I have better luck this time. If the mixed build doesn't work I'll try just a debug one again to see if there are the errors I had before, or if not so that I can compile tests and see what might be failing.