It's anecdotal, so not sure how true it is, but I have made a fair profit by finding a system that is high pop, anarchy, with a local pirate faction with high influence (or even majority in some cases), seems to increase the rate of pirate vessels, including higher rates of the larger ships. Sweet spot for me is the dangerous/elite viper/cobra/asp range, which net up to 40k per kill, but a fair few larger ships will wander through, (although the time taken to down a mostly-harmless anaconda for about 10k isn't really worth it - the 90k elites make up for it, along with cargo drops)
i also have my pet 1t block of palladium, which i have named 'bait' to suck in any potential pirates from factions i want to remain friendly with, just gotta remember to let them shoot first.
occasionally the rate does drop off, but fsd'ing out of the nav beacon and then re-entering seems to reset the instance.