What in-fiction reason would a Deadly pirate have for not going after a fully laden trader flying through his space just because the pilot is Mostly Harmless? Crazy.
Okay imagine you're the NPC Pirate, what's you motivation?
The Deadly Lone Gunman might well take on the 'rich but mostly harmless', but might expect an easy fight & do it in a smaller ship.
Or maybe they have bigger fish to fry so they go after a higher ranked target & leave the Mostly Harmless Trader because they 'probably can't afford to fill that ship with cargo'
I think pirates that interdict you in quiet (population zero), or well policed systems would be largely doing so because they were forced out of areas with better pickings, so they are poor, not that good, and just looking for scraps. 'Your cargo of 2t of food is worth a fortune to me'.
One that has better skills may have contacts in the Smuggling outpost that tip them off about a tasty haul taken from the mission board, and set a trap where they know no Police will be around, because they are experienced & don't want Police around any more than the smuggler does. They know who they are expecting and what ship & combat rank they have from the tip-off. They are busy setting the traps so don't have time to bother the trader carrying who knows what?
So the decision to interdict is based on intelligence (intermediate to high-ranking foe), or desperation (low ranking, cheap ship).
An Elite NPC Pirate is a successful Pirate Lord & doesn't need to to the dirty work themselves, they must be hunted down with a trail of breadcrumbs by assassins.
Also, I think a Pirate is going to use a ship that can carry enough cargo to make it worthwhile. I think this would mean they'll sacrifice SCB/Hull reinforcements for cargo space where for example an NPC Bounty Hunter might not. A Pirate would be more likely to use a multi-purpose rather than a combat ship. If they use fighters (I'm thinking about Vultures here) they will have say a T-6 or 7 in their wing to collect the goods, but if the cargo ship is destroyed they can't make any money so they'll abandon the attack to regroup.