I think I have a good suggestion, I don’t know if someone already mentioned it, but sorry if so. You can download Moonshell, it should come with a folder called dpg tools, inside just open dpggenc.exe. You can convert mp4 files to .dpg files, using it, and then put the Moonshell folder and the Moonshell.nds file on to a Micro SD card that you put into a Flashcart such as R4. You also want to put the the videos on the micro SD card, you can put them in a folder called videos. But, whatever video you put on there, just open it with Moonshell on the flashcart. You can even convert and watch movies, so yeah there is a lot.