I don't think Biosphere 2 is a great example. In Mission 1 there were huge problems regulating CO2 and O2, their pollinators died, invasive insects took over,

I don't think Biosphere 2 is a great example. In Mission 1 there were huge problems regulating CO2 and O2, their pollinators died, invasive insects took over, and they had continually increasing NO2 and CH4 gases. They eventually injected O2 from outside to make up for the decreasing levels, and when a participant got sick, they smuggled supplies back on their return. It was a disaster. And they had a door they could open to walk out. The issues with getting something like an ecosystem on Mars would be much more dramatic, esp. given that while there are some ways of getting O2 and carbon, there isn't a source of nitrogen on Mars that we know (we assume there's fixed nitrogen in minerals, though that's very hard to breathe). Any loss of significant life forms would need to be replenished from Earth. And while some life forms will probably manage in 38% gravity, there's no reason to assume many would have long term viability (especially vertebrates like humans who depend on Earth gravity to function). Getting an ecosystem on Mars that could support humans doesn't seem like it's in the realm of possibility to me, especially when there's good reason to believe humans themselves wouldn't be able to survive for long periods there unless something managed to solve that very hard problem. You could use a centrifuge for the long trip out (and nobody could survive without that), but you can't just live in a centrifuge on Mars forever.