I would second flatpak and app images, not everyone is on NixOS, and I would like to not use NixOS any time soon as it doesn't appeal to me the way it works.
YES THERE IS, THERE IS A TOOL FOR LINUX THAT TURNS KEYWORDS INTO WHATEVER YOU WANT THEM TO BE, I just need to find it again so hold on FOUND IT, IT'S https://espanso.org/
If you plan on using windows only for games and absolutely nothing else then there isn't much of a point in making a shared partition between the two OS'es. Just keep them separate, to each partition its own.
(So your first example win 100gb, Linux 400gb is what I personally would go with)
I would second flatpak and app images, not everyone is on NixOS, and I would like to not use NixOS any time soon as it doesn't appeal to me the way it works.