A rambling thought on the situation
Exploiting cheap SAD for caravan safe travel could be reduced by tying the amount paid to the duration of the “fleeced flag.” For this to work though, the payoff would most likely have to be in goods instead of coin, since moving the goods would be the point of the caravan. In order for coin to be viable, something would have to be lost in the transition - if the bandit is the merchant’s alt it does not matter how much the merchant pays him since the money changes characters, but not players.
As an example, this means that if someone wants to move adamantine ore between settlements and theoretically needing to pay 50% SAD charge to insure a “fleeced flag” for the entire journey, they would need to leave half of it behind with a fake bandit should they desire to game the system. Since the merchant now reached his destination with only half of his original ore I think this would fall under the lower risk, lower reward.
Granted you could keep reusing the same ore to stay behind (if they could insure they could continuously move that same amount of that specific good through one settlement), but eventually you are sitting on a lot of expensive material that has not done you much good for quite some time. It also is not a viable tactic when you are trying to transports it from the original harvesting node since you would not want to leave half of it out in the middle of the wilderness.
This would also present a meaningful choice for the bandits engaged in a normal SAD. Do you ask for a lower percentage and be able to carry the spoils on your person? Or do you demand so much that you need your own cart(s) or caravan to transport it, with all the logistics and risk associated?
This would probably also mean that the effect of a SAD on a caravan would have to be handled mechanically different from one on a character not linked to a caravan - and may require separate flags "robbed" vs "fleeced" or the like. At some point the number of flags gets excessive though.