while i agree that the system needs to be changed to balance it more, your suggestion would simply kill plundering entirely.
people can convoy 4 times, they spend 10 ingots to get an extra ss convoy, that means at the cost of 30 ingots i could get 240 coupons a day as a strong player who currently plunders, 240 coupons - 30 ingots > 150 coupons
plus if people make alts on low pop servers just to plunder their main now your suggestion would give them extra free coupons, 360-30>150 as well.
this just destroys the convoy/plunder system entirely, no one on any server would plunder with anything but *t made to lose to their main.
a much easier solutions to fix the issue, remove costs to do multiple ss missions, if f2p can do 4 ss a day then thats 60 coupons minimum 120 max, which makes it balanced compared to a plunderer getting minimum 0 max 150. and even if a convoyer gets plundered they have a support to try and help them anyway, while plunderer is solo