the dreamcast fishing rod is probably one of the more unorthodox controllers out there, so because it was designed for very limited use (and is pretty obscure), most mapping programs such as joy2key don't natively support it. maybe if you were really dedicated you could hire a technician and give them a lot of money so they could analyze and design an adapter for it, but that would probably be excessive.
if you want a more difficult touhou experience, there's always arbitrarily imposed limitations and ultra patches. not sure if touhou 15 has an ultra patch yet, but there's the black label version with scarcer resources and faster bullets