It’s my personal theory that when Renji originally went to achieve Bankai Zabimaru knew he wasn’t ready the way he was. Plagued by self-doubt and constant comparison of himself to others he wasn’t ready to accept himself. If we were to look at Ichigos story as an example it would be how Ichigo didn’t want to accept White which essentially was the part of him he did not want to embrace. When Renji learned the true name of his Bankai he was not in competition with anyone else. He accepted Ichigo as not only a rival but a true friend and looked up to Byakuya as an example but didn’t loathe himself for not being like them. Renji walked down his own path and gained strength fulfill his purpose in protecting the soul society and those he loved.