Ok. I've made some progress. Qt5 plus qtwebengine are built. There was some error when trying to compile the KDE patches and I didn't look at it at the time. Trying the canonical edition, same error, ended up being an issue with compiling on Monterey:
https://code.qt.io/cgit/qt/qtbase.gi...b1b3680e34a547
Qt Patches: some as you know are already in 5.15.x, others or at least one the changes were a bit different, one seemed to also be already patched. I'll try to collect info or retry once I get closer to complete or it's done.
Trying some of the prerequisites, from what I recall xz didn't compile, though I haven't looked at it. I think most of them were fine. Some I had installed with Homebrew though may go back to redo later.
Trying to compile Python, is it possible, at least for this compile, to use a non-embedded Python? Maybe that'll be a secondary option. For Apple Silicon, as you can guess, the target would be 11.0 which I believe is supported with Python 3.8.12+ (patches were backported). That's compiles fine as well as sip 4.19.25 (unsure if correct version).
Stuck on compiling PyQt5. A quick look, it's hard to determine from the change log which version I should use. Maybe other libraries have some mis-match. Still investigating.