For Intel graphics, it’s good to rule out if using “triple buffering” can resolve (or partially improve) such video artifacts and lines.
Here is the contents of my custom /etc/X11/xorg.conf.d/99-intel.conf
Section "Device"
Identifier "Intel Graphics"
Driver "intel"
Option "TripleBuffer" "true"
Option "TearFree" "true"
EndSection
I use a combination of TearFree and TripleBuffer.
You need to logout and login for it to take effect, or preferably reboot after making the changes.
I can only vouch for 60Hz and 75Hz refresh rates, as I do not have a 144Hz (or higher) monitor in my possession. Not sure how this can translate for multiple monitors, or if you need to define a Screen section.
As @Fabby alluded, you can run a test without the dock or any externally connected monitors, and see if there’s a notable difference by using the laptop monitor exclusively, as a process of elimination (with and without the above option for triple buffering.)
Since you’re using KDE, another place to look that can be used in conjunction with the driver settings is under Settings → Display and Monitor → Compositor
You can adjust the Latency and Rendering Backend.