The common trope is specifically the opposite of what I'm saying. In the common trope the previous life entirely subsumes the existence of the current life because the previous life usually just had a larger existence/experiences. They rely on the common sense and expectations of the previous life because fundamentally they are the same person they were in the previous life just in a different body.
The impression this story gave me was that the kid entirely absorbed the previous life. This is not an adult in a kid's body, this is a kid in a kid's body with some extra memories and experiences. Ageha even refers, on a few occasions, to the memories from the past life as being a separate entity from themselves. Chapter 1 even specifically says Ageha devoured the memories of his past life. In chapter 3 at several points when referring to something from the past life and it's relation to him he uses "I" instead of I. Making a clear separation between the him of right now and the him of the past life. Ageha expecting kids books to exist for no reason and thinking the low education level being a disaster despite what Rottenmeyer is saying is inconsistent with this portrayal. These two things only make sense if the previous life version of Ageha was the main controller or he is arbitrarily unable to separate himself from these bits of knowledge despite it not being a problem previously.
I'd say you misunderstood.
In the first chapter, page 5, he states that his mental age is that of a thirty year old. On page 9, he states that he is no longer his old self but this is ambiguous. Does he mean that he is no longer his previous life? Does he mean that he is no longer his kid self? Does he mean both, that he is no longer the kid nor the past self?
As you say, from chapter 3, we can clearly see that he does denote a difference between his current self and his previous life.
From that, my conclusion to the previous ambiguity is that he's both no longer the kid and no longer his previous life; he is something new, a product of both memories and experiences existing in his current body.
I will add that, in chapter 3 still, he is shown having trouble going through the memories of his kid life; it's all hazy to him and a smaller part of his current self.
So from what I see, he is very much following the common trope. If he was still a kid, who simply had knowledge but not a sense of self imported from his previous life, he'd still be throwing tantrums, he'd remember the recipes and think to recreate them because he wants to have a new experience of tasting this food from his memories. He'd act in awe at the taste because it'd be a new experience for him. But it's not, because all the experiences from his previous life are now a part of his current self and of course they'll have changed him. He has consistently not been acting like a kid from the very beginning.