An update on this: I installed the #Fedora 38 KDE spin and this problem went away. It just worked.
On the first boot, the default display settings already had my external 4K and built-in laptop display scaled independently.
No blurriness. Everything is super crisp. Settings are retained after a reboot.
Still not sure what's going on with Ubuntu/Kubuntu regarding this issue, but I'm SUPER happy that Fedora seems to solve it.
ONWARD!