Unfortunately there isn't a magic bullet approach to this, and it depends largely on what configuration you have carried out in your time with Mint KDE.
As you have surmised, any KDE or user application level configuration is in your
/home/user directory, and it is great to see you have had the forethought to hang on to this. Often, particularly where the distros and DE are the same, you can just re-use the entire
/home without changes. Be sure to keep a copy of the vanilla one (that is created before you make any changes) so that you can figure out the differences if anything goes awry. The most important thing here is to make sure that your user UIDs on the Mint KDE and Kubuntu distros are the same. You can view these in
/etc/passwd and if it doesn't you many need to
/home/ user folders to the right userid:
sudo chown user:users /home/user -R
user is the username and
users is the primary group.
The majority of system level configuration exists in the /etc/ directory. Unfortunately, you aren't going to be able to just copy this over, as you will break lots of things. For each system level app configuration, you are going to have to take it one step at a time, and decide whether you can just copy over the
/etc/ file for the app in question, or will need to merge changes, or re-configure as you did the first time.
Applications will also store information and "configuration" in
/var/ - MySQL databases, including their structure are store in
/var/lib/mysql. Cron files are stored in
/var/spool/cron/, websites in
/var/www, and the list goes on. If you haven't used any applications that store data in
/var/ then none of this matters. If you have, then you may need to copy directories over from
var too - there is no way to say without more information.