Lucci and Kaku are nowhere near YC1. YC1 still applies to Zoro as it's a general term used to refer to the number 2s of Yonko crews. Technically, Mihawk too is a YC1 now lmao
YC1 is a fan made term that has nothing to do with power levels. The power levels in manga are boutny classes, and Mihawk is a 3 billion bounty class while Zoro is a 1 billion bounty class.
Just like with Captains in Bleach, JOnin in Naruto, Power Level in DBZ and Nanantsu no Taizai, and every other class system in every other manga, Bounties are a power ranking system that can be tweaked for humorous effect. At the end of the day, they are just numbers buttt the writer uses them for effect. YC1, 2, 3, 4,6, and so on DO NOT EVER appear in the manga, vivre cards, anime, movies, and so on.