Have you tried https://github.com/B00merang-Project/Windows-95 or any of the similar themes by that project? There's a bit of extra padding still but it's vastly more usable than the Adwaita default. (And of course you don't need to bother w/ the retro icon packs shown in the screenshot, the basic GTK+ theme is plenty enough.)

(The best UI with that kind of widget style today is arguably SerenityOS, but the repo I linked above tries to provide something similar for existing GTK+ apps.)