The original term for a ruck was a loose scrum.
So why don’t we treat it similarly? It could be called a ruck with with two players, one behind the other. But if a third joins at the back the ref calls scrum (or loose scrum ) , allowing the SH to come down the side but not being able to contest until ball comes out as per normal scrum.(Neils ,I think you made this point about coming down the side in a previous post). The ball could be picked up by the third player or the SH as per a normal scrum.
At least this would limit the length of the caterpillar to two stretched out forwards and SH or a much quicker ball to rear and used.