Help Finding Alt Formal Attire

What it says on the tin. BIL’s wedding will be in September and I’m having a hard time finding a dress that checks all of the boxes: fits my style, “formal” enough while not upstaging the bride, fits my size/shape, and isn’t fast fashion. I had a baby 10 months ago and I’m a curvy gal, 43/35/46, so inclusive/flattering sizing is a must. My style is pretty diverse to multiple alt subcultures, so goth/witchy/punk/grunge would all be relief to be able to find.

Are there any brands/sites y’all know of that might have what I’m looking for? I’m bad at formalwear so I’m desperate 😭

Edit for more context: We’re a pretty progressive family so I’m not worried about the appropriate-ness of my style. For reference, this is what my MIL (mother of the groom) wanted to wear if it weren’t fast fashion: https://www.coramoon.com/products/green-snakes-in-witchy-floral-forest-printed-hollow-sleeves-maxi-dress