By way of a follow up, I've discovered a way to make the game work in the way I want it to
I'm now flying an engineered Krait Phantom. It has the agility, cargo space and jump range to make it a good cargo mission transport.
Even with fully engineered weapons it's a challenge against higher ranked / larger NPC ships. I'm also running 3A shields (to max the cargo space) and lightweight hulls to improve jump range and agility. (So I'm crippled myself a bit, but for good reason rather than 'because')
I then stack a load of boom delivery missions and await the sweet sweet interdictions

If I have several missions in one system, often other ships will jump into the instance whilst I'm fighting the ship that interdicted me first
By doing this I've also noticed something that's kind of awesome: If I have to run from a fight, when the NPC interdicts again, it now has the same rank, and shield/hull damage. i.e. it's the same ship I was fighting before. Was this added in a recent update?
Kudos to FD for that: it used to be that NPC ship stats reset, and sometimes changed rank too with each interdiction
Having fun again