Since the word bridesmaid, however archaic that might be, does not mean a boy or even a ‘whatever I am on the day’ the simple answer is don’t be a bridesmaid.
Nowadays, a bridesmaid’s only role (unless you have an older one to support the bride) is to look pretty behind the bride and in the bridal party photos, they quite often wearing similar dresses.
I’m not sure a suit in the bridesmaid’s colours, depending on what they are would be any better, as that implies something equally binary.
The girl could wear whatever she is comfortable in and be an usher and just not walk down the aisle behind the bride, because that would focus attention on her rather than the bride and I’m sure she wouldn’t want that.
I wonder, if you take out the non-binary issue, and a potential bridesmaid said she didn’t want to wear the outfit the bride has chosen for her bridesmaids, what the response on here would be.
It’s not about being a bridezilla, to choose the outfits you’d like your ‘attendants’ to wear on your wedding day. My youngest brother wore a kilt because I asked him.