Since there is no manga indication or logical reasoning to claim that kakashis has 1/20000th of itachis power an his kamui is tiers below itachis katons as feats show means that base itachi beats him comfortably let alone MS itachi who not even kaguya or JJ Hagoromos could beat
Their fight in Part 1 had Itachi ahead, with Kakashi struggling to keep pace. Kakashi got negged because he believed in the sharingan hype to protect him from genjutsu.
Their fight in Part 2 was a more balanced engagement. Itachi had an edge due to Susanno, but that's his last card.
War Arc Kakashi was trading blows with godtier characters and Bijuu. While Itachi lost to Hebi Sasuke.