what makes you say he is not transscendant?
i say he is because we know kenny is poor at controlling reiatsu and hence the eye patch was created for him....furthermore kenny is the shinigami with the most
reiatsu in SS...further more no shinigami can compress and hide their reiatsu to the point of non existance hence urahara the genius created his reiatsu cloak for the reason of hiding reiatsu.
so he is transcendant or as stated, yachiru is only imagining him
Well considering it's fact that a transcended being is someone who surpassed the levels of both a Shinigami and a hollow which only 2 characters have ever done because they had hollow abilities.
Kenpachi is only a Shinigami so no matter what he can only be a Shinigami so he has limits to his power, then there's the fact we have no idea what Kenpachi's Shikai can do, so we don't know if that is what caused his spiritual pressure decrease, also it's a fact as well Transcended beings are like 3D beings while everything around them is 2D cause they can't touch them unless of course they purposely bring their power levels down to a point where others can detect them it makes no sense as to why Kenpachi would bring his power level down.
Unless we get some explanation as to why Kenpachi's spiritual pressure couldn't be sensed we can't just assume it's because he's become a transcended being as it would contradict what Aizen himself stated since a transcended being is someone who has surpassed a Shinigami and Hollow.
The problem in your theory is the amount of inconsistencies in them and unless they are explained I'm just not buying Kenpachi is a transcended being.