Oi nah, I like how you actually have an intellectual approach to this.
Look the way I stand is, akainu showed very little power against a SICK, DYING and near useless whitebeard. I do agree that his endurance/durability is probably the highest in the series, but a lot of characters have some pretty endurance crippling movesets. It took 3 admirals, X amount of vice admirals, countless fodder and even the emotional blow landed by his own crew mate (that probably shattered a bit of will tbh but that's a lil head cannon) to take him down and akainu wasn't even the person to deal the final blow. Against the likes of Kaido I feel like akainu doesn't have the strength for that kind of fight, against the other ones he has a higher chance but that's mainly to do with match up. You could say being able to push any yonkou to high diff would make him yonkou level but in reality the only yonkou we saw him even close to half matching was fucking dying and akainu still got pranked. I love akainu for reasons and I hate him for some, he is definitely one of the strongest fighters but man put it up to feats not headcannon