They probably have several generals above Kanmei's level at their disposal.
The only generals they have > Kanmei are Kou En and Renpa, I doubt Chu is just littered with Kanmei Level dudes lol, most Chu generals are probably Rinbukun Level given that Karin was elevated to prime minister over the rest of the Chu Generals and Kanmei >= Karin
But I don’t think Riboku will be the final boss. He doesn’t fit that role narratively. I think the final boss will undeniably be
Lord Shouheikun, The Last King Of Chu
Riboku is meant to be the opposite of the type of General Shin wants to become. Shin has so far idolized valorous Generals Who personally lead their troops from the front, and fight with ambition, whereas Riboku is a calm tactician who doesn’t share the dreams of people like Ouki and really doesn’t even like warfare at all, as he said in Bayou.
Once Shin becomes a General, or at least becomes GG Level (now?) then Riboku’s roll in the story will be over as Shin should surpass him.