I completed everything and got all the trophies for the DLC, I just need to get one more piece of clothing and it's done. I'm going to probably do the new game plus with all this extra stuff next week as I never even finished that mode so I might as well start over.
Overall the DLC was great. It's short but the overall plot adds a lot of backstory that wasn't shown in the main game. It's mostly about Jin's father.
There's not too many good vanity pieces to use for Jin, but you're able to unlock some of the online skins for story mode. I was expecting more sword skins but there's a little less than ten and most aren't great.
You can travel to the highest point in on the island at a mountain peak and you can see all of southern Tsushima in the distance, and opposite of that I'm assuming is mainland Japan.