We probably do not need to wait for ideal planets position to outrun Voyagers. We have ion thruster now with much higher specific impulse!

Voyager-2 gained about 10 km/s at Jupiter, about 5 km/s at Saturn, about 2 km/s at Uranus, and lost about 2 km/s at Neptune. [1]

Dawn spacecraft gained 11.5 km/s from ion thrusters.

So just gravity assist maneuver just around Jupiter alone + massive tank for ion thrusters gas might give 20+ km/s

Though I agree that value of such a mission would be low.

[1] https://www.planetary.org/articles/20130926-gravity-assist