Mizuki's story absolutely screams transfem and it makes the most sense to me, but it's not wrong to consider them non-binary. It's not confirmed either way and we don't get what they're preferred pronouns are cause that doesn't really exist in Japanese.
Assuming their pronouns are "she/her" and that they have to be transfem is just as presumptuous as saying they're 100% non-binary, both interpretations are consistent with the story and acceptable.