I moved this to the #virtual-reality-vr:vr-wishlist category, where it is now voteable.
Note: There is an existing wishlist post for a slider for brightness in particular:
https://forums.flightsimulator.com/t/i-wish-to-have-a-brightness-slider-in-vr-mode/342994
There is also a wishlist topic in #self-service:wishlist for such sliders applied across the entire game:
https://forums.flightsimulator.com/t/options-for-contrast-brightness-gamma-hdr-sharpness-level-color-saturation-etc/320448