And what happens when your manager’s response is “you’re the expert, I expect you to decide these things?”

This industry is like any (every?) other: a minefield of incompetence and laziness interwoven with an unhealthy dose of unjustifiable arrogance.

In some ways, that might be a good situation -- it means they are leaving all the trade-off decisions to you. If they are only specifying time and resources, you can trade scope, quality, debt.

If they are freezing time and resources and scope, and you dont have much room to move except with quality and debt. But those catch up. If that is the consistent trade-off, i'd go with my original recommendations: trade this stupidity for more salary or find a new employer with better management. Or seek the managerial position yourself and do a better job at it.

Always provide options and a recommendation.

If this doesn't work, then I think the most important thing is to implement the most simplistic basic solution as fast as possible and then explicitly ask for feedback. Hard-code whatever portions that take time, but you're not worried about (e.g. database portions). Focus on the parts that are vague that you don't understand and provide a _concrete_ realization of the those vague ideas. This shifts decision-making responsibility away from you.

As a plus this also gives you the ability to more effectively gauge the difficulty of the rest of the project. The things you thought were simple might be much worse. And then as long as you get concrete feedback, you'll be able to much better predict the amount of time and effort required going forward.