A lagrange point outside the moon system won't shorten the distance at all.If it cuts the distance in half it may still be worth it - depending on what's there. If we explore it and there's something really valuable then the turnaround time matters a lot.
The lagrange point is at the same distance from the Sun as the planet is, but 60 degrees behind in its orbit.
... so the Sun, the planet and the lagrange point all form the verticies of an equilateral triangle. The distance from the lagrange point to the planet is not any shorter than the distance from the Sun to the planet. No distance is saved.
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