ls -t is clever, but in general I’d probably lean away from just ls-ing in the store.
I made a suggestion at How to find which package dependency it is? - #3 by waffle8946 of how to deal with this scenario, basically slapping --show-trace 2>/dev/stdout | grep 'while evaluating derivation' on the rebuild command might give you better answers.
Also to the overall thread, packageOverrides is long deprecated, I’m surprised it even works half the time… IMO should be removed from nixpkgs entirely.
You’ll want to use overrideScope instead if you still want to go ahead with the overlay:
python3Packages = final.python3Packages.overrideScope ( ... )