I think OP described it pretty well - stock Gnome is actually a nice desktop environment, I prefer it to Windows, does not even come close to MacOS.
I can believe that KDE Plasma still has issues though, it seems some sort of bomb was thrown during KDE 4 times in the codebase and it still has to recover. Though if nothing else, i do use KDE apps outside of Plasma (Kate, Krita, Dolphin, Okular, Spectacle) and they all seem very solid.
After a parent said that Gnome looked better than Kde I looked it up and found a number of posts criticizing Kde design decisions (padding, etc). I fully believe the Gnome people have a better design decision. But I don't think that criticism applies to Qt itself, necessarily.
It’s consistent enough that it makes me wonder if maybe something about the tooling is partially to blame.
The UI designs are just a byproduct, there is nothing about Qt that makes it hard/impossible to have Gtk-like designs, but since Gtk is used by a lot of programmers, when encountering a program like those you mention, the chances are higher that it is written in Gtk than in Qt because more programmers use Gtk than Qt.
(note that this is specifically about Linux desktop applications, i think overall Qt is used more than Gtk outside Linux)