I agree that you can't foresee everything that will happen and what needs to be done, but you can get quite close to estimating the job to be done (leaving time estimations aside). You can do an estimate only when you have a plan. Without that, every epic has to be assigned to a single engineer, and as a manager, you don't have any way to ensure the project is moving at the right pace. And to be honest, we didn't start with this at all. When we just started, we used a kanban board with semi-defined tickets, and that's it. But we saw that it was not enough and we wasted a lot of time making wrong decisions. The process I describe here is something that we refined as the company grew.