I just finished reading Journey, and it would bother me a bit if you don't at least explain what's happening
Even though the story seems like classical fantasy, I'm very curious about the war and that people in that country have no idea about it, and I imagine the ending could either be a simple "government hiding things" situation, or a super cool plot twist. And the possibility of the latter and not ever knowing what it could be, feels disappointing
There is some good advice here already, so I'm just gonna post my personal idea. I'd use some key illustrations accompanied by explanatory text to tell what happened from there and conclude the story. Maybe leave the characters (it's a good point now because they just teamed up and left to solve the mystery) and use a third person narrative, maybe from a historian's point of view, or even one of the characters but in the future, to tell what happened.
Or if you really don't want to spend that much time, you can even go with the found notes that say something in the lines of "I don't have much time, so I'll write down what you need to know, what we found out when blah blah..."
Don't know if it helps but sure hope you find an alternative that leaves you satisfied, even if it's not what you had originally planned.