I live in Japan and bought a few packs of BBM sumo cards in the early 2000s, but could never commit to it (decided to stick with baseball since its what I know best, though I'm a sumo fan too. Happy to see Hakuho get his 43rd championship the other day).
Ryan here would be a good person to hit up if you haven't already, he's probably the most knowledgable person on the subject of sumo cards on the planet (and does trades):
https://sumocards.blogspot.com/