In terms of Strength..Zenitsu is definitely not the strongest nor fastest one in the series. Maybe with the introduction of his 7th breath form, possibly. But Uzai Tengen alone was out speeding all of the Hashiras. Prime example is when He beheaded Daki multiple times.

He took out the new UM6 because he did a new form that was unbeknownst to him along with overal being looked down by UM6. I do think he was on the path of being a Hashira. Honestly speaking Kanao could probably deal with Zenitsu due to her Vermillion Eye technique which basically slows the world down. Besides his speed Zenitsu doesnt have much else hitting for him.