For reference, I've done a lot of killing pirates - like around 7,500 hrs worth of it out of my approx 15,000hrs total game time. This is my experience:
Even in a weak spawn, the pirates come in waves, which seems to match what you're saying. I can judge a spawn within a few seconds of arriving, so if it's not the type I want, I relog until I get it. That's what I do to reach at least the top 10 commanders whenever I participate in the bounty farming CGs. It seems to beat all other methods.
I did a lot of tests on how to optimise AFK farming for pirates when it was a thing a few years ago. For that, cargo is essential. The trick for success is to get the right spawn too, but you need one not too busy and not too quiet. I'm pretty sure that having the cargo made no difference at all to the type of spawn, which is under the control of RNG.
One thing that does increase the number of pirates is having a partner. When my friend does it with me, there are always more pirates. I don't know whether that is only when you have missions, though I think not.
I've never noticed any difference in spawn rates at certain locations during the 10 years I've been playing, except maybe when there's been a change of state, power, allegiance or something like that. None of the places where I do it regularly seem to have changed.
