What about fingernails? Would cutting a fingernail without consent be equal?

Absolutely. Let's switch away from fingernails to hair because that's something I can talk about with person experience. I have long hair, plenty of people have jokingly threatened to cut it in my sleep or such. To have my hair cut like that would impart no physical injury or ailment to me at all, but it would be such a severe violation of my bodily autonomy that I would have no reservation about considering it assault and bringing charges as such.

I think your point is that fingernails are just a bit of extraneous keratin that is universally removed as part of grooming and so the violation cannot be equal to having your entire arm removed, but perhaps you forget the many women and some men out there who like to decorate their fingernails and that this is an expression of self.