I usually see べき nominalized as べきは. Is it grammatically correct or just a casual way of saying? How is it different form べきことは?
べきは is a more pompous, not casual, way of saying べきことは.
What we should now is wait.
べき is the attributive (連体) form of べし, and using it like a noun is basically an old way of speaking Japanese, although this is found in many patterns of modern Japanese:
That said, べき is already a stilted word, so the difference between べきは and べきことは is not very large.