For UI/UX designs the GTK design team has some interesting guidelines here: https://developer.gnome.org/hig/stable/
There is a repo in Gitlab with basically all GTK items here available for GTK mockups: https://gitlab.gnome.org/Community/Design/
Please correct me if I am wrong: in GTK there are standard widgets (dropdown, radio buttons) that, depending on the GTK theme the user has on their OS, have different styles. Using the standard widgets is probably the most straightforward way to develop a new feature without specific design development.
Not sure if too crazy because it changes a lot of stuff in the UI, please get it as an example of how this settings might look following the GTK design principles. I've mixed the current work of Marién with the comments of Jabier and the work of the gnome design team. Sorry if its not correct, I made it really fast.
I've used a standard settings mockup, couldn't find the dark one, but I think is ok.
I've also attached the SVG if useful
Thanks for this work, is really useful :)