Edit: Yep KDE is the most customizable full desktop environment. I gave Gnome a shot but like KDE A lot more with desktop effects and kwin scripts.
What makes Linux appealing to me is the extent of customizability, but I didn’t find many answers when looking up with desktop environment is them most customizable. Some say KDE is most customizable than say, Gnome, but doesn’t Gnome support CSS customization while KDE doesn’t?
It seems like you’re thinking about “customization” as “customizable look and feel”, but “desktop customization” goes way beyond than that.