As you know, I reckon, you can’t schedule publishing to same category, because it is… already published 
Actually Discourse doesn’t have scheduled publishing. It has scheduled move to another category, what is called as scheduled publishing.
Bad UX, I think.
But I’m quite sure the team doesn’t change that too fast (they have tendency listen you, though ) so use it more ”expected” way: publish it first in staff-only category and schedule it with moving to users-reply category. Problem solved.
If I now understood right you…