In my experience, theming KDE Plasma is an absolute no-go. Not only do the themes themselves just look off, but the desktop feels so much clunkier. Themes also ruin consistency on Plasma, making certain windows look like patchwork.
On the other hand, Breeze (Plasma's default theme) does grow on you. It's like Plasma has some weird spell about it, once it even caused me to prefer light mode! Gah :O
Somewhat unrelated, but by the sounds of it gaming is a focus for you considering the DE jump. I've had the best gaming experience on a distro called Nobara, I think they started shipping Plasma by default for the same reasons you switched. Could be worth taking a look at, if you aren't familiar with it. Obviously it's a perfectly good day-to-day distro too, it's based off Fedora and follows their release schedule closely.