It’s good you got the bride’s input because it’d be quite likely for the reverse to happen, I would show up in cocktail attire and everyone else would be wearing barn party & cowboy boots.

So I do thing that your search will include cocktail ( nit va- boom, but dressy), wedding guest, festive, party. I don’t see how you will avoid synthetics for those, though of course you might find 100% silk. Also I would of course try to find cuts & styles that are “ you” and would get the closest to what you think fits & feels great in this more formal ( but not Formal), whether or not you have a lot of occasions for re- wearing. But you might get lucky with something that’s “ right” and also versatile.

Taking a chance here but I’m wondering if raw silk might be a fabric that combines your style with that of the wedding party? More formal but still ‘natural’ and textured. It’s a fabric I associate with Thailand and you could consider having something made overseas via Etsy or the like - a bit risky but you have time if it doesn’t work out.

I went looking for some examples to explain what I meant and came up mostly empty handed so I feel the need to give some context. I’m not at all suggesting you wear Thai traditional dress, but when I travelled there they made a variety of western styles for tourists out of beautiful raw or dupioni silk - which was also maybe much more in style when I got married over 20 years ago, but now seems harder to come by in terms of examples. My sister was working in Thailand at the time and had a lovey pink strapless dress made to be my bridesmaid.

I do think that if you like natural fabrics, Etsy is worth a look for reasonably priced made-to-order items. Just make sure to read the reviews and keep expectations reasonable in terms of tailoring quality.