Thanks a lot! This finally did what I want:

widget.non-native-theme.scrollbar.style = 4

widget.non-native-theme.scrollbar.size.override = 20

Though I'm not sure if I maybe want:

widget.non-native-theme.scrollbar.style = 3

widget.non-native-theme.scrollbar.size.override = 30

Plus of course that, which I had found earlier and did nothing on it's own:

widget.gtk.overlay-scrollbars.enabled = false

layout.css.scrollbar-width-thin.disabled = true