There's a reason we can't reliably provision a generation ship, and it's that we have never yet tried to completely and permanently sever ourselves from Earth's ecology and biosphere. We may think we've got it all covered, but if there's a leak in the cycles somewhere, or something missing we never knew was important, our intrepid astronauts are going to be in for a hard time, either immediately or generations later.

So redo Biosphere 2, for longer. Even the first time it was done, it worked remarkably well! They did make some mistakes but recovered, and came out healthier than they went in. That suggests a multi-generation-capable version is not as far off as one might pessmistically conclude. The most valuable information is always the first information - if a problem doesn't appear quickly, then it probably isn't that important...