there's no secret sauce anymore, there's nothing to figure out.
those days are long gone.
all you can do on Switch 2 is lower the settings, lower the resolution, cut the framerate in half, and then use DLSS to improve the image quality.
there's not much more in the cards.
if they wanted to make a good looking Switch 2 port of GTA6, they'd need to replace their RT GI with maybe a coarser probe based solution, and turn off RT reflections fully, only use SSR and dynamic cubemaps (or best case only the dynamic cubemaps tbh)
but the next issue after that is the game size, which seems to come in at 200GB

which is already insane on a PS5 and Series X... and super insane on a Series S.
on Switch 2, that is a HYPER INSANE size.
so they'd probably have to downsize it as too. lower texture resolution dramatically, reduced audio quality, maybe if any of the lighting is baked they'd have to reduce the quality of that as well.