Maybe this is really flawed, but I feel like its a fairly simple fix.

First: Lower the loss. If people are only losing 10 coupons when they get plundered, they're probably not as likely to feel like its a waste of time.

Second: Insurance. Make it so that if a person pays for an extra convoy they can't get plundered. This way people don't feel like others are profitting off their hard earned money. They can still get plundered on their free one though.

Third: Make it to where if a persons support defeats at least 2 of the plunderers characters, then the plundered only loses 5 coupons. This way support isn't totally useless if they lose.

If they do this I bet a lot more people will convoy, which means there will be more people to plunder as well. Everybody wins.

Do these changes just for convoy and I bet a lot more people will be happy with the system without even having to mess with plunder. Because honestly, you shouldn't have to. Plunder is like a reward for being one of the strongest players in your server. Its something to strive for. (I feel like I should probably point out that I am far from being able to plunder. So this isn't biased.)