No question here is dumb! This forum exists to help people with questions exactly like yours.

What you're planning to wear is perfectly fine and you will fit right in. I have occasionally been in F wearing short pants, tee shirt and boots (with socks!), looking like a genuine dweeb, and I have never been admonished, questioned or looked at twice by anyone.

Occasionally I see men my age and above who like wearing a suit and tie. And their female companions dressed up as well. Clearly retired people who prefer to dress that way. It's their choice, they obviously feel more comfortable dressed in their finery. If you see an F passenger like that, do not feel under-dressed!

The dress code is fairly broad, but I personally would not wear sandals or have bare feet. I don't feel comfortable with that, although I often do see sandals on passengers in F!

Enjoy the flight!