Meh, his agenda has always been this depressed, bored pirate because nobody will truly fight him, not seeing how this is supposed to change.
First off, we need to stop assuming that Nasujuro follows the same "rules" of swordsmanship that black blade users follow anyhow. We already know that Black Blades upgrade the ranking of the sword as it was originally. This implies Yoru was not a surpeme grade before it turned black. Shusui is a level lower as a black blade, which means it was even lower when Ryuma started using it.
Shodai kitetsu (what we assumed Nasujuro has) is the only surpeme grade blade currently known outside of Roger and Whitebeard's swords (and maybe Shanks' Gryphon). There is no confirmation that Supreme Grade swords can turn black on their own. That said, we still do not know what the upper limits of these Gorosei are. And considering Saturn's showing already, its pretty high.
Mihawk is relative to every other endgame character here, that being Akainu, Shanks and Blackbeard. Lets stop inflating his "greatness" over characters clearly more important than him (i.e. to Luffy, who is the MC of this story)