Trying to bottom-center an item in a column, but it keeps left-aligning

Align is the way to go if you have only one child.

If you have more, consider doing something like this:

return new Column(
  crossAxisAlignment: CrossAxisAlignment.center,
  mainAxisSize: MainAxisSize.max,
  mainAxisAlignment: MainAxisAlignment.end,
  children: <Widget>[
      // Your elements here
  ],
);