I know I'm late in doing the KDE upgrade, but was in the middle of creating many resumes and job apps and just didn't want to screw up my system when update-notifier first started reporting a repo problem a couple of months ago.
I get the "E: Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages" error message when following OP's instructions. Except that first, when I hit Mark All Upgrades, this message comes up:
To be kept:
There is an option to Cancel and one to Mark. I click on Mark and then get the "Error: Resolver, and Unable to Correct Dependencies" message.
But if I try to fix broken packages, the message just repeats. There is no possibility to uncheck the above "to be kept" packages from any list. APPLY doesn't generate a list of packages to be upgraded, it just brings up a message: "Could not apply changes. Fix broken packages first!"
Sorry, I'm beyond my depth here. Any help appreciated.