> BTW for an example, here is what the KDE system settings looks like for me when I try to run it using qt5ct in GNOME: https://i.imgur.com/pxEKXye.png

I don't get why you'd want to run KDE system settings outside KDE. The system settings app also has a lot of optional dependencies. I don't know which distro you're using but if any of those optional dependencies isn't installed, the system settings app misbehaves.

Typically, you'd run apps like Spectacle and Gwenview and Dolphin, standalone apps that serve a purpose, and from my experience, those have always worked on i3wm without issues.