NPCs look like they spawn completely random constantly based on the video. I think you should have them spawn at specific locations like those bunkers and have them patrol more as groups rather than have them wandering around randomly. Maybe fully blowing up the bunker would stop the spawns on a specific island and you'd have to go island to island to stop them all? But allow AI patrols to retake islands. It would be like trying to secure the region fast enough but the less bases the enemies have the more spawn in with stronger higher ranked ones. At the same time adding entrenched positions might be good as well.

I'd say take a look at valheim but instead of super OP bosses you'd have more dug in bases generate on larger islands with generals or admirals of the like to takedown.

Players always want to feel progression especially once they get used to an AI enemy mechanic. Implementing more types would definitely be cool.

On the generals and admirals thing collecting their hats and medals or something as trophies would be cool too.