Two things worth mentioning. There are very limited ways of earning XP, basically quests and POIs / camps. On the map page, at the bottom right you can see how many POIs you've completed on each biome so you can max them out and get all that sweet sweet xp.
The other one, and this is a big one, is to always have some building mats on you. Boars don't seem to break through walls, so when facing multiple, you can box some of them in and face one at a time. This trivialized a fight I had to do many times over.
Question - ship combat - do you always sink the other vessels, or also board them and wreck them? I've only tried boarding two vessels. One from my tiny little dinkyboat, and the other was the first ship for the quest. On both occasions I was fighting a seemingly endless supply of dudes and eventually gave up. Is there a trick to this or just keep killing them until they stop?