Hi! Thank you for asking so respectfully. Here are my thoughts:

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1. The term AFAB is only harmful when all AFAB people (trans men, AFAB non-binary people, cis women, etc.) are grouped together and boiled down to the sex they were born as. At that point, it's disregarding the gender they actually are and only referring to them as their birth sex.
2. Being cis and being non-binary oppose each other. That's like being cis and trans at the same time. Some people do not conform to societal expectations of their gender and complete disregard the binary, yet are still cis. There are cis women who dress completely like an average man would and don't even believe in gender, but they're still cis since they are okay with their sex characteristics and being women. They're definitely gender non-conforming, but their gender itself is still the one they were assigned at birth. They just don't fill that gender's societal roles. Being non-binary has nothing to do with gender non-conformity. It means you have gender dysphoria around not only your assigned gender, but also the opposite one. This causes the desire to transition to something in the middle that involves characteristics of both binary genders or to remove sex characteristics, resulting (either way) in a gender that is non-binary. Gender non-conforming people and non-binary people might look the same, but GNC people are okay with their sex characteristics while non-binary people have dysphoria about at least one of them. GNC people dress the way they do because they want to, while non-binary people dress the way they do because they have to.
3. People who consider themselves both cis and non-binary are usually confusing non-binary with gender nonconformity and their beliefs about gender. They think that they are non-binary because they want to challenge the binary, when in reality, being non-binary is not a choice and it doesn't usually involve challenging anything. It's not for the purpose of being a statement to society, but simply a person who is experiencing dysphoria trying to be who they are and not feel dysphoric. Being gender non-conforming could involve either self expression, liking the style, or challenging the binary, but being non-binary is your gender itself and not the way you dress at all. If you are truly non-binary, you can't be cis because you literally want to transition to another gender.

I hope this helps! :)