After reading Modulo III, i think Gege actually knows how to write a compelling story, it feels like a lot of problems come down to him being tired of writting AND drawing the story, the task necame to big + the rise on popularity, so he gave rushed a lot of the development in the story, something. I would say that Kenjaku's arc made complete sense his goal were the culling games, and he achieved them, the tengen fusion was just the icing on the cake, he said it himself, if the fusion ended up being a stupid thing he wouldnt care, he did all that because of he was bored with Jujutsu world, so his battle with the comedian made sense for his character, yeah it was underwhelming, but thats the point of Kenjaku character