That act of breaking up and organizing tasks of the same size, that's what estimation is. This feature has these tasks, historically we complete these tasks in

That act of breaking up and organizing tasks of the same size, that's what estimation is. This feature has these tasks, historically we complete these tasks in this time, so here's a hard minimum for a completion date (which implies cost). Slap on an appropriate fudge factor for dealing with other teams, testing and burn in, and general error bounds as needed. You've described scrum, what you're doing is scrum.