4 hours of 100% CPU… it finally finished. It was just a few changes to about 12 files… so the test TC mod eventually compiled successfully to 2.7Mb in size.
A second cook resulted in another 10k shaders compiling… and the entire LandscapeSublevel.umap.z (421Mb) cooking for no particular reason… and an eventual 971Mb mod.
It was a fresh install of the devkit… then I overwrote the files with the Github 232.4 files. Maybe the trick is to re-verify the install (roll back to the current version)… cook the TC… then overwrite with the latest 232.4 files and try again.
After a re-verify, the devkit came to 232.4 already. TC cooked worked straight away with no shader compilation.