So here is a thought: what is the bride wearing on her feet? That might guide you a bit. I mean, in my mind, even by today's casualized standards, "semi-formal" doesn't really include sneakers. Or galoshes. Or, really, any footwear that you might normally wear or bring to a beach, LOL. But maybe I am wrong about that, in which case, things are easier.
So here is another option and maybe it would work: these Cole Haan Original Grand stitchlite wingtip oxfords. You can find them on sale in various places. They come (or came) in different colours. There is also the Zero Grand (not quite as comfortable, for me, although some people like it more) and a flatform version (also not quite as comfortable, but not bad. Maybe a bit clunky for your purposes.)
What I like about this shoe: it is incredibly lightweight. You could easily switch it out for the brunch and put it in a bag that you leave at the venue or in your car. It is washable in the washing machine so if it gets all dirty that day, no problem! It is closed toe/ heel so could keep you warmer -- but it is not hot/sticky/sweaty. It is quite refined looking on the foot, even with a dress it looks good (to my eye -- see photos below.) So if there were beach photos, not too big a deal.
Maybe you will need heattech undies beneath the pretty dress.
These people obviously run far warmer than many of us do. I mean, I am from Canada and even I am shivering at this thought....
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