A better approach to solve for this problem is to completely ignore, at least for the moment, any idea of decision fatigue and instead focus on developing a solid plan. A good plan will account for risk acceptance, business requirements, a task list, and costs. The most valuable part of a solid plan is not having some magically complete roadmap or blueprint, but that it puts all your challenging decisions up front. It forces you to examine what you are willing to pay in terms of money, time, and effort.
Once you have a well envisioned plan the only consideration of decision fatigue should be limited to maintenance. If a given maintenance effort is too expensive or requires too much investigation or risk then you need to refactor and break things down into more primitive units.
When it comes to running a team there are only two things to consider heading into planning: a defined performance baseline and scope of effort. Those two things are completely in competition and you have to balance them against a budget of available resources. As the team leader be willing to accept input and suggestions from your team, but set the baseline with confidence and don't let the children run the daycare.