Honestly, just reduce the advantages to plunder and the convoy/plunder system would be better.

-Currently -
Gets SS Convoy, Gets Support, Gets Plundered. Took out 2 Ninjas or heavily damaged the plunder. Plunder beats support. Plunder gains all their ninjas/health back to full. Your line up comes in, gets faced smashed in if have a lot lower power.

-Suggestion-

Get SS Convoy, Get Support, Gets Plundered, Take out 2 Ninjas or Heavily Damaged enemy team, Plunder beats support. Your Line up Comes in, beats Plunder's Face in (Hopefully)

(In other words, don't heal the * plunderer between mostly cause there is no downside to plundering considering 50 coupons is a bigger profit compared to convoy SS)