What does one wear for a business lunch with some big shots in a strategic industry? Informal, business-casual, smart-casual?
I tend to dress conservatively for conferences and lunch meetings. A high quality dark suit (navy, charcoal gray) with a good clean fit. I wear pants here, but if outside the US I might wear a skirt at or just below the knee. A single strand of medium-sized pearls, tasteful earrings (diamonds or white sapphires), and a good watch - then men see the business side first and foremost, and yet you have not sacrificed your feminine side completely. My hair is quite long now, so I tie it back. Makeup in a soft neutral palette. Complimentary shade to fit your natural tone for lipstick - not too pale, but not hot pink, screaming red, or goth purple-black...
I have never found that this is "overdressed" for these occasions and as a uniform it is like a fortress.
Of course, there is a cultural bias here that responds well to this kind of "language", but it might seem much too formal and masculine for your context.