My advise and highly recommend --- Start From Scratch
Yes, sucks monkeys, but, should be good for future for awhile (maybe/maybe not)
You have years of that home folder (kde4) you say, well kf5 is new, no time like the present to begin new, things are bound to break now/soon/later
This and other things are the very reason I never keep a separate /home. It's too easy, less time consuming to just backup my important configs, etc, to another drive, ftp, cloud and not to ever deal with transitions/change/whatever.
You could also start new, slowly one by one stick configs in your new from old, time consuming, but, you might find the problem
You can also wait for someone else to chime in and give advise