That’s not even the biggest issue. Every zoan we have any real data for has three different modes. Their original form, a full transformation into whatever “animal” their zoan represents and a hybrid of “human” and their fruit.

Even other mythical zoans like Marco and Kaido have that.

Luffy is different in that regard. He has elements of his fruits even in base. Is Gear Second supposed to be hybrid? Or Gear Fourth? And is Gear Fifth his full zoan form?

These aren’t huge issues, but they are interesting to think about.

Replying to a 6 month old post, but...
Luffy we see just walking around is not transformed at all. Anytime luffy is stretching, he's in the combo form. Than there's gear 5.
I don't remember the old fleet admiral having a combo form, but I'm not sure if it really matters. There's rules in one-piece, but they get broken. It's not as bad as Jojo in that regard, but it's still a thing