This is a small outpost but with several targets and alarm as well. You can climb the water tower southeast of the island to have a good vantage point to scout and a convenient zipline that will take you to the back of the facility without getting detected.
There’s a lone sniper on the roof but he’s not going to be a problem as long as you take out the cultists outside his field of view. There’s a ladder behind the facility that will take you to the roof. You can use that to get rid of the sniper.
Like before, pick off the lone patrols and if possible hide their bodies. You can deliberately leave the corpses outside to lure their allies but avoid attacking them while they’re nearby each other to prevent them from getting alerted. The facility has a lot of cover you can use if ever enemies will start looking for you. You can even flank them from the roof using the ladders and the walkway inside the building. As long as you tagged them (or you brought Boomer with you), you should be able to pick them off silently by tracking their movements.