For me the plunder is not the problem it is the convoy that need changes.
You keep the 3 plunder with 50 coupons rewards and you make 4 ss convoys (30coupons) per day without having to pay 10 ingots.

The players that want the highest reward possible (also the highest risk) go plunder with a chance of getting 0 to 150 coupons per day.

The players that prefer doing convoys gets from 60 to 120 coupons per day.
it may be necessary to reduce the coupons from convoys a little to keep plunder interesting.

With a lot more ss convoys plunderers will not have to spam refresh fro 3hours waiting for convoys to plunder.
Oh and people will be less salty when they get plundered. That's a good thing too.