There is a reason project management calls scope-cost-time the iron triangle. It's extremely to bind all three sides tightly unless the estimates are very good

There is a reason project management calls scope-cost-time the iron triangle. It's extremely to bind all three sides tightly unless the estimates are very good, and/or there is significant added buffer to the project (and that really means the cost side is what's stretching to give you extra certainty on the hard scope and time deadlines). More commonly at most two of the three are tightly constrained, and another side is more free. One can optimize in some directions, but none of the sides are fully independent of each other. I think the best project outcomes I've seen on are ones where only one side is strongly fixed, and the team is in control of the other two aspects.