Thank you OP, this was a nice learning experience.
Well worse case scenario, I only had to re-install from media, then restore the config xml file. as I've done in previous failed major version upgrade.
but is good to learn how to re-cover from a bad upgrade.
Almost every major version upgrade like this 2.3.4 to 2.4, or 2.x to 2.3, and all previous major number changes, seems to have problem upgrading properly.