Tom

For me Gnome and its GDK4 widgets feels like waisting space. Also many window title bars, button sizes and margins are way to big for me.

You seem to be aware of many things.

Just for communication you are using more unfree tools then free and more then in the other categories.

A question with no right answer, because it is a matter of taste and habits.

Both are very much sophisticated desktop environments which very useful defaults. Neither Gnome nor Plasma are too complicated for beginners and can be customized easily. If you want to you can go very deep into the customization too.