>>287003656
I think you'll need to create a custom surface shader with a material attribute to index into a texture with all the tile faces on it, which unity will then convert into a vertex stream for you for the gpu instancing thing. Maybe there's an easier overall way to do it though. I briefly tried unity's sprite packer thing, but I think that requires their sprite renderer.
>>287003931
I think I have the logic of mahjong down, but if you're keen, you should open unity and mahjong yourself and try to code it up and post results. If you run into any weird udon bugs first, it'll be good to know about it ahead of time.