I finally wiped out all of core and noticed some things (that others probably already knew about).
Accessibility is at maximum, since my colonies are the only ones left.
I have many mercs and freelancers I could hire. Too bad I cannot hire more.
Pirate activity is almost non-stop. I guess since there is no one else left, pirates target my colonies. I wipe out pirate bases, and they respawn and target my colonies days later. With non-pirate core worlds, my colonies only get targeted few times per year.
With no export income, income per colony is maybe less than a tenth than with everyone around, less than 100k. Even with only pirate and pather core worlds left, I could make about 200k-300k.
I guess no matter what I do, I will always babysit something.
It seems that if I really want to stop all expeditions from major factions while leaving enough core worlds for pirates to torment instead of me, then I should take a commission for the one major faction I should spare, preferably one that controls the most worlds or systems. Hegemony is probably out if they do inspections even if commissioned with. Maybe League, they are as big as Hegemony. Then again, Tri-Tachyon does not care about atrocities, and they have more markets than Diktat. Diktat also has lobsters, which means if I kill major factions, I could take out Diktat and take over Volturn to control lobster exports. Then again, taking over Volturn probably means wiping out Nortia and Umbra to stop extermination attempts.
If I simply want no inspections (no bribes, no chance of core theft), then wiping out Hegemony only seems like a good idea.
I guess if I really want to kill everyone, I probably should farm alpha cores then build many colonies for more income from population before going on the extermination crusade.