I've done some more testing. If I put rename YSMenu.nds to R4.dat, I am able to boot YSMenu and load my roms just fine, so I know there isn't an issue with the flashcard nor the microSD. If I place the generated YSMenu.ini file onto my flashcards microSD then launch my flashcard it will autoboot the game that's set, so I know the YSMenu.ini file is being correctly made.
When I launch from a forwarder, it loads the NDS logo and Health & Safety screen (I set that to enabled to be sure it was reading the ntr-forwarder.ini file) and then those disappear and it hangs on dual white screens. It does this even when I have YSMenu.ini on my flashcards microSD card.
If I rename YSMenu.nds to TTfwd.dat and try using a forwarder, there is no change.
So from what I can tell, it's either failing to load TTfwd.dat or loadcard_dstt.nds is not working right. I can't narrow it down any further because even if I completely remove loadcard_dstt.nds there is no change in behavior. Perhaps there could be a check and message presented if loadcard_dstt.nds fails to be loaded? That would at least pinpoint where the issue is.