Functional Product Management cycles through working versions of the solution to a problem in short cycles, daily or weekly, possibly monthly. If its longer than months, the problem is too hard and needs to be broken down.
A product is a solution to a problem. Identify the problem, decide what is in scope, and more importantly what is out of scope. Solve the problem, scale it and win.
Managing a functional product involves iterating over working versions.
You don't wait until the end to deliver a product. Work to launch your product today, solve the problem today, and learn from that experience and the product you make tomorrow will include the learnings of today.