Absolutely nothing? Have you tried maybe talking to them before you just plop something down on their land? Cause it sounds to me like you're sh*ting where you eat ><
As a solo player, I recommend you find a nice renter's island, or somewhere that is generally friendly to squatters. A short conversation with the island owners can go a long way. After that, just GO ELSEWHERE to do your pirating. Never a good idea to piss off your neighbors, especially, as you say, when there is a giant red circle showing them where you're hiding.
Many renter's communities (as others have mentioned) are quite active/strong. Often they have a community discord for communication, trading, and mutual defense. I think you might be confusing solo mmo play with single player play. Just because you don't want to be in a company doesn't mean you shouldn't utilize a community.
Being a solo player requires thoughtfulness in the way you play, build, and INTERACT with those around you.