JFem, I don’t understand why knees are attractive myself. The most elegant length is just past mid calf, IMO, though I wear longer.
A lot depends on minhag hamakom. Generally, to feel comfortable anywhere, women should keep their collarbones, elbows and knees covered; I do feet too but there are communities that are more liberal and I don’t mean MO.
I think this a CYLOR issue, and everyone who considers him/herself observant should have a rav/rebbetzin/mashpia to ask questions of and bounce things off of.