If both options cause problems, you should pick the one with the best workaround. Sunglasses can reduce brightness, but increasing brightness is much more difficult, so dark text on light background is correct.
No, sunglasses aren't going to fix contrast sensitivity due to photophobia. As a low vision person putting on sunglasses inside just means I can't see anything.
It’s possible to get polarised lenses that have about 50% transmission which is quite comfortable for computer work. At least for me they are anyway.