xahodo Would it be prudent to switch to another admin account and delete all the kde stuff, for this user, in /var/tmp and switch back, or will that result in a borked user?
I don't know. I am not a Plasma user, and I don't pretend to understand how the start-up configuration files work and/or relate to one another in the Plasma DE environment.
If I remember correctly what I read at this morning, though, Plasma users have resolved similar problems by deleting the /var/tmp/kdecache-username files -- using TTY while Plasma desktop was not running -- without a problem, and the configuration files that were rebuilt when the Plasma DE booted again resolved the issue. That is far beyond my capabilities to evaluate as a solution -- as I said, I'm not a Plasma user and have no way to test the solution -- but frankly, that supposed solution sounds like a good way to bork the DE. In my hands, the probability of borking the DE would probably be close to 100%.
I did notice (reading between the lines in the threads I looked at) that the problem is one that develops over time, as Plasma users try this theme or that, change the configuration this way or that, over a period of months/years. I suspect that what happens is that the number of configuration files build up in the directory over time -- one thread reported that the /var/tmp/kdecache-username folder was over 1 GB -- some active and others not, and the files end up conflicting somehow. But I need to point out that I don't know that -- it is just a guess.
Something you might do before you do anything else is to take a look at your /var/tmp/kdecache-username folder and see how many files are in the folder, and what the size of the folder is at this point. If the folder seems unusually large and/or you find a lot of files that are from now-unused themes and so on, it sounds like the files might be conflicting.
I suspect that the method you are suggesting -- create a new admin user, letting the configuration files set up for that user, deleting the configuration files from the old admin user, and then switching back to the old user and letting the configuration files rebuild for that user -- might work as well as TTY deletion and subsequent rebuild method I saw on the threads I read. But I don't know. In this case the blind man (me) is talking to the one-eyed man (you). Your guess is probably better than mine.
BTW, I didn't suggest reinstalling out of my butt, exactly. Sometimes a clean reinstall is the simplest, least time-consuming way to resolve a complicated issue that has no obvious cause and no obvious solution. In this case, if nothing else, a reinstall will start you out with a clean slate. If the problem is what I think it is, chances are it won't repeat.
I ran into that decision-fork the other day, with Ubuntu Budgie. I had UBB installed on a 128 GB disk in my computer, but needed that disk to set up a tie-etching system (don't ask) on a surplus computer at the railroad. So I swapped an older 256 GB disk with UBB installed on it into my computer and ended up with a boatload of ACPI errors on boot. After an hour and a half of trying to figure out what was going on (I can be stupidly stubborn about that kind of thing), I came to my senses, wiped the 256 GB disk and did a clean install. Not an ACPI error in sight and I saved myself several hours of frustration.
It is up to you what to do at this point. Whatever you do, back up your data. That way, if you try to fix the issue by cleaning out /var/tmp/kdecache-username and bork your current install, or go ahead and reinstall clean at this point, you won't lose anything of importance.