The other, I find easier option is to simply have a single condition: $session.params.last_para_to_be_collected != null is the easiest way to route via a condition.

This works no matter how many params your collecting as it only routes when the last != null.

The required parameters means cx steps through in sequence, only moving on when last_param != null effectively gating the route till the last required param is filled.

That works for me anyway.