You know, in this particular situation...

Need a lightweight jacket
need it to be really smart-looking
limited time

I might bite the bullet and go to a tailor. Take pictures of argyll and braemer and crail jackets. Take lots of them, take pics of the darts in back, take pics of the pockets, etc. If you can take in an existing jacket that's heavier weight, but in the style you want, it'll help. Think hard about what sort of material you'd like. A light suiting wool is a good possibility,I'd also agree with linen, though linen is a serious stinker to keep looking crisp. It wrinkles at the slightest opportunity.

a light summer-suiting weight plain wool blend might be the ticket.

This won't be "cheap". But on the other hand, when you're done you will have a great jacket for summer, and you will be able to wear it for decades, as it will be the furthest thing from "disposably trendy" than anyone can imagine..