Seems like some of the problems with Biosphere 2 would have been easy to fix.
It could have been designed to operate with half the sunlight, or even a quarter, with shutters to block the excess, or larger species populations to consume the excess.
The concrete could have been sealed to prevent oxygen absorbsion. Just keeping the oxygen levels up would have been a huge benefit.
It could have been designed for excess, like nature. That's why we see boom and bust cycles in the real world. There's room to grow, and when nature fails to provide such abundance the overgrown populations die off, leaving a small number of survivors. The cycle begins again. Natural selection.
I wonder what other problems there were?