How do I design systems under tight constraints?

You can design most systems well. It is the ones boxed in by tight, competing constraints that defeat you. That is not a gap in your ability. It is the hardest kind of engineering, where every choice costs you something elsewhere. Constrained design is not about finding the perfect answer, because there is not one. It is about finding the least-bad trade and being clear-eyed about what you gave up. The engineers who do this well have made peace with the fact that constraints mean compromise. With your next hard problem, write the constraints down and rank them by which truly cannot bend. Design to the top few and let the rest flex. You will not escape the trade-offs, but you will stop fighting the ones you were never going to win.