Maybe it could be done without adding extra steps.

Blend could work the way it does now but if you click a certain distance (5% maybe) from the endpoint this would be the starting point.

Left and right sliders could make adjustments to those points or you could use direct controls in the viewport to adjust start curve and target curve contact points.

The sliders and controls would not appear if you click near an endpoint.

If you don't adjust any of these the command would work the same way.

Would this make some sense?

Marc