Layers/outfits that work with a johnny-collar sweater-polo like this

Going to a wedding late May in Seattle and have been instructed that the dress code is “garden attire”. I like this sweater-polo and I think the idea would be to pair it with navy wool trousers (so, darker than the ones I’m wearing in the pic). However temperatures can fluctuate quite a bit throughout the day and I am wondering what a good layering piece would be — a blazer doesn’t quite seem right to me due to the thickness, but I am open to inspo!