Buzz will probably take care of it but it might get you up and running sooner if you don't want to wait for the fix to push through. I think 64bit is probably is the right flag to use here; looking at the other modules it's only used a couple of times, but it is used outside this particular module, while x86_64 is not (not as a module_flag, anyway. Its use for other things like as a platform identifier in this same module is still fine, I think.)
@dankcushions Thanks for the quick response. Kind of sad that it's the effect of video transfer and not an improvement to the game itself. But I'll try that configuration change to see what it does for the game! I appreciate it.
@derekr resizing to 512x512 seems to have fixed a lot of the issues. Another one that I came across was that the ".PNG" extension of a couple of my files was capitalized. Make sure that your files use the lowercase ".png" extension.
Aha! I see it was failing to find the BIOS - I didn't consider that because literally every other game has worked perfectly. dc_boot.bin and dc_flash.bin were both in there, but they were the wrong files. Have now replaced them and everything's peachy. Cheers for the tip :)