This. My ex-husband and I had a medieval themed wedding. Custom-made costumes for everyone in the wedding party, including the horses we rode in on. Historically accurate foods. Wandering minstrels. A red dragon ice sculpture. Hay bale seating. We encouraged everyone to dress according to the theme. Even my grandparents, very conservative traditionalists then in their 70s, put together fun outfits that looked fantastic and blended well. Some people went all-out, some less so, but nobody showed up in a suit. To me, that would have been highly disrespectful. Don't want to costume, fine. But don't stick to some antiquated idea of what a wedding "should" be when the bridal couple has made it crystal clear that theirs isn't that.
To the OP, I think the outfit you've described is perfectly fine unless the invitation says black tie.