>but you need to have a skill that produces value in such short projects.

I assume you're a software developer? There's your skill.

I think you're overthinking this. How is it less agile? You have a project, a price and a timeline. Everything between then and the end is on your schedule.

The only "unintuitive" skill you need to hone is assessing value and pricing based on that. But there is endless analysis and analogies out there to help.