Lazy vs Clean turnaround at an end node. Lazy turnaround would make the tug immediately start pathfinding towards node 0 after pathfinding to the last node, and clean turnaround would make the tug attempt to execute a turn of equal radius to the length of its barge train, preventing jams.
New tug modes: Cycle and Gofer. Cycle mode would behave like it does right now, moving from node to node seamlessly, pausing at docks. Gofer mode would wait at node 0 until called by a signaler or other stimulus, then pathfind its way to the node that called it.

For Lazy vs Clean turnaround there isn't really a point in doing that, since we don't treat Node 0 any differently than other nodes (other than the fact that tug routes start at node 0).

If you want the last node to smoothly transition to node 0, you can simply add more nodes at the end.

@Violet-Scarelli a question about the Gofer:

Why not instead dock the tug and force it to wait using a redstone signal. It can the be released from the dock using wireless redstone/said stimulus. Is there some scenarios where this solution isn't good enough?