It'll be easy - once. Those JS and DS are also incredibly valuable to the periphery merchants or states. If you start losing them going to Backofassawhompset IV to trade, you'll just stop going there - it's not as if periphery merchants have a space navy/police force to call on, true?
You also mention the other problem - to a pirate with a Trader and a Union, a couple of ASF could represent damage to your fragile JS. And you then have to assume you can capture the other JS intact. Otherwise you've lost your working capital, and your ride home.
No doubt about the value of the merchant vessels. I figured that would be why you do not want to take everything, just the cargo if possible. Pirate attacks would still be enough to ward off traders for a time, perhaps long or short, depending on all kinds of reasons. And that's another reason to stay on the move - aside from wishing to stay out of reach of the anti-piracy forces that might seek them out one day. Certain of the Houses have been known to cull the pirates from time to time, but we know that can come at strategic cost along other borders. By staying ahead, one can skulk into other, less-suspecting pastures and allow the site of your last raid to return to normal.
(edit: I'm thinking years for things to settle. I would see the need to manage my territory, plot out loops that take you through a long circuit, plot contingency loops and more contingencies on top of that. Perhaps some places never see you again after you make off with a dropship full of particularly valuable loot. Returning to the scene of a crime is bad news, they say, and I can think of why that would be too. We all could, that's a standard battle on the table; waiting for the pirates, and *poof* here they come... )
Aside from that, I would also bet that most pirate bands are too self-interested to ever attempt a seizure in space, and for great reason. The cost of failure is loss of your assets and likely death by all kinds of means, from the hazards of the undertaking, to capture by vengeful enemies if you really botch the attempt.
But the old Redjack Ryan color fiction piece from The Periphery indicated that such a move could happen, and it inspired me to think that it might have a chance to work given the right set of circumstances and a lot of luck.
The ASF issues are mighty formidable, too. Most pirate groups do not seem to be all that ASF savvy, that it's kind of a luxury to have even a handful of flyers. So it would definitely be a perquisite to an attempted seizure, if simply to combat the ASF forces of the defending vessels.
Risky business for sure. Agrarian planets with no outside communication and no defending forces of note are so much more welcoming. :)