When I start my DSTT card, the screen goes way darker than it is on the menu, and stays that dark when I start a game. This doesn't happen on my "legit" games. How do I solve this? Increasing it on the DS menu doesn't work.

The TTDS saves its own brightness setting and utilizes that setting. If you are using YSMenu, there is an obvious button in the upper corner to press. If you are using TTMenu, then you should be able to find it in the SysOptions.