As the title implies, I can select the first case, but after that it just sticks to a black screen. What's the issue here? I have my donor ROM as Mario Kart DS if that matters.

I may be "smart" but I'll be straight with you, I don't quite understand, sorry... :/nds-bootstrap may have not patched all of the card read code in this game.

The card read function reads any data from the Slot-1 card.I may be "smart" but I'll be straight with you, I don't quite understand, sorry... :/
Ah, thanks. In the compatibility list it says it does work, but tested on an earlier version of nds-bootstrap. (https://docs.google.com/spreadsheets/d/1M7MxYQzVhb4604esdvo57a7crSvbGzFIdotLW0bm0Co/htmlview#gid=0)The card read function reads any data from the Slot-1 card.
nds-bootstrap patches the function to instead read from a ROM on the SD card.