KDE is actually lighter. And hidpi is much better on Qt.
[1]: https://addons.mozilla.org/en-US/firefox/addon/auto-tab-disc...
LXQt if you want something with less features, Plasma if you want more features. They're both performant.
As for Wayland, I use it on a Raspberry Pi 4 (Raspbian xwayland). It works well enough, but there is some kind of bug that causes xwayland to fill the swap space at almost 100%. When this happens, I find it necessary to close applications to reduce the swap usage.
Not sure what you mean. We have CSDs available for people who like it, but they can be turned on and off.
If you're talking about random GTK apps that run on Xfce, there's not much we can do about that; we don't set app policy.
That's one of the nicer things about Wayland, though: there's a way for the compositor to tell applications "I'll draw the decorations". On X11 there's no protocol for that, only a way for an app to tell the window manager it wants to draw its own, without a way for the WM to say "no, don't do it".
Yes, guess you're correct. Qt apps in Plasma was a better mix for me. I have consistency and no funky titlebars.
But I don't get the HiDPI point? Certainly 4.16 had issues with HiDPI, but in 4.18 you can set the UI scaling factor, and I believe shortly before and/or after initial release, we fixed up all the issues of blurry image rendering. I have a HiDPI screen and it's worked very well for me, and looks nice.