>>718292667
Play on the hardest difficulty.
You don't need characters to guard everything, focus on rebuilding your current region.
One good first steps: After getting back to the continent, explore the area, gather materials, rebuild the harbor. You should be able to do that without getting into the next mission. Go back to the island, do the missions here, garrison the villages. Also, buy the runic sword. They are great, the best investment with your meager money in the beginning.
Check the tower in the witch swamp, after you get Yahna.