In fact, a trained eye will notice that between Whole Cake and Wano,
Luffy gained confidence. So much so that I remember mentionning how Oda started to draw Luffy in a much more demonic way in this arc, with an expression close to the mischievous kind of demonic face he now depicts in Gear 5.
To quote a few...
And this confidence grew until Luffy started to enjoy the very action of fighting against Kaido, much like Goku.
This is not random.
This is the expression of Luffy slowly synchronizing with the very essence of his fruit both in term of personnality AND power.
Gear 5 is more than just joy and freedom, it's
a demonic and mischievous loony toon kind of joy and freedom.
So it's not that Luffy is not Luffy anymore. It's that Luffy became more than just serious, he started to enjoy fighting, which in turn woke up his fruit and made him transform thus emphasizing this trance.
It doesn't mean that Luffy does not go to the maximum of his capacity, it simply means that he doesn't look the fights the same way anymore.
This is what Luffy's journey has been leading up to from the start.