The difference is we’re not even focusing on Imu. We’re focusing on Gunko who’s Imus avatar for now.
We are, he blew up an island, we know he wants to flood the world, its literally him speaking through gunko, its just the design, and by the time the design is revealed the fight happens right away. Hes been building up to imu constantly, thats why oda keeps harping on about joyboy this and that. goroseis been fully revealed, holy knights revealed, regen weakness getting exposed, all set up for imu and gorosei to be destroyed the next arc to finalise this imu saga. Once imu is defeated the joyboy narrative is no longer relevent, blackbeard wont even mention it.
While oda continues to build the conflict between luffy and imu, blackbeard hasn't even killed shanks yet. Shanks being a figarland means hes relevent to the imu saga, so oda cant even get rid of shanks until the celestial dragon saga is finished first