That's lovely, I think that would be fine for a wedding. I've only ordered from Seraphine once and it was while I was pregnant, so can't help there. I found it a little on the long side but I am very short so I think that's my problem rather than the company's.
Have you had a look on the high street? I'm also breastfeeding and have been to a couple of weddings recently, I had a dress from Oasis with a cross-over front which I just wore a little cropped vest underneath - it was similar to this: www.oasis-stores.com/silk-botanical-bird-dress/dresses/oasis/fcp-product/4470118460. Oasis have loads actually! This one is nice: www.oasis-stores.com/blake-crepe-dress/dresses/oasis/fcp-product/5550069833. Monsoon and Phase Eight also have plenty with wrapover fronts.
Not trying to put you off the Seraphine one, I think it's very nice, but just wanted to throw out some other options. Two weddings and a Christening in, I am now expert in dressy breastfeeding outfits!