You see, scheduling in a project is always context-specific. Make this clear to your customer. If it is a project that primarily corresponds to the first or second scenario, you can plan the project from start to finish. However, if it is a complex or uncertain project, you must work your way through to the goal together with your customer. Yes, you must proceed iteratively, constantly reflect on whether you are still on the right track and work transparently and openly. This is the only way you will succeed in adequately reaching your customers.
This approach is not specific to start-ups, but should also be practiced in established companies. Finally, one should always keep in mind that projects are unique by definition and project management is pioneering work. Consequentl israel telegram data the methods and procedures used must always be adapted to the situation, especially with regard to planning.
By the way, you don't have to stick to one planning strategy for the entire project lifecycle. If the clouds of uncertainty disappear after some time in the project, you can plan the project through to the end.