I've tried Cosmic recently and it's glitches galore right now (on nvidia at least). I think safest point is KDE. The most familiar paradigm, mature wayland support with mixed refresh rate displays, HDR and other modern features that XFCE can't do.

> the default designs tend to feel old-fashioned, giving a false impression that it won't do what you need

Who is actually getting this impression? What thing that they "need" is in doubt?

> I guess I assume "BS" means "UX flourishes that most end users are used to,"

You assume incorrectly. Every OS and DE finds some way to be obnoxious, even when you've learned the tricks and keyboard shortcuts. XFCE just seems to have the least of them. It's predictable. I think a new user will be able to navigate it immediately. I don't know about KDE, but I sure couldn't say the same about Gnome 3.

For older machines I'd recommend Mate. It's a fork of old Gnome 2, so it got a lot more polish back on the day, even though some of it bit rotted away.

It's still a very nice desktop and you can combine it with Compiz if you want to have some fun.

> The settings feel like you're almost in a config file.

What on earth?

No, the config has dialogues and intuitive controls. There is a settings-editor you can go into if you need to, with a bit more of a regedit kinda feel, but I haven't looked in there in years.

> Gnome or Cosmic are safer starting points.

In Gnome, can I move the UI elements to locations I want them in? Or are we still in a situation where it's opinionated and you have to seek plugins to get an experience that you actually want?