Holy shit, double-checking this actually sent me in the right direction!
I missed it because it’s not in the normal picture settings, but Menu > Input Settings > HDMI-1 > Full HD Color… defaults to Off, but when turned On, 4K@60 is now available in KDE Display Configuration! 🤦♂️
Thank you!
It was somehow a TV setting that I’ve never changed in this process. Post updated.