I've been using Xubuntu for years - it just works, with no clutter. Absolutely my only complaint are the very thin window borders - this is a setting you cannot individually adjust. However, one quickly gets used to (alt + right-click + drag).
Seems to be a major problem in many desktop environments. I use the "super + right-click" in Gnome for the same reason (Ubuntu).
On GNOME you should be able to click in the area near the window borders (e.g. in the drop shadow) to select them and be able to resize the windows.
Alt+space brings up the window menu. Press R to resize, or M to move the window with either the arrow keys or by moving the mouse pointer. Finalize with Enter, cancel with Escape.
Can you resize a window with only the arrow keys? For example hit r+up and raise the upper border upward, then s+left, and shrink the window by moving the right border leftward, or something like that?
Yes. After pressing Alt+space then R, the first pressing of an arrow key selects the border you want to move, let's say the left. You can then move that with the left/right arrows. If you press up or down in this state, it will, again, select the corresponding border to work on.
XFWM themes support different thicknesses for the window with focus vs those without. In my custom theme, the focus window get a thick accent-colored border all around, and non-focus get a 3 pixel black/white/black border that guarantees a visible separation without drawing much attention.
How amusing, the thinnest possible window borders and using Alt+drag is a feature I desire and intentionally wrote myself:
https://github.com/superjamie/onepx
The default Xfce theme doesn't seem to have that issue anymore, at least in 4.18.