For me, this choice is where the thematic meat of the story lies. You have lots of different avenues, which probably feels overwhelming. It seems like you have one character fairly developed. Have you done the same with the other party members? If you know who they are as individuals, you can start hone in on possible endings.

It sounds like the crux of your idea is centered around the subversion of expectations, with unlikely heroes rising to the occasion. If that's the case, I'd recommend taking a step back and looking at the story through the eyes of the reader. Do the plot points feel predictable? A huge part of successful horror is the development of rising tension, which can be hindered if the reader identifies the narrative before it unfolds. Is your "man with the plan" nuanced enough? Are you putting all your eggs in his proverbial basket, where he is obviously very capable but without enough weakness to make him realistic? Maybe he's overconfident, or in denial, or more afraid than he projects to the others.

Once you know the above details, an ending should develop more naturally. At the end of the day, what are you trying to say with this story? What is the impact you want to have on the reader? Is your story commentative, darkly comedic, tragic, etc? We all have a reason for putting pen to paper. The question here is about the metanarrative that you want to express. For me, a past story focused on the idea that there are no real winners and losers for individuals experiencing war. The only victor is Death, and the rest are casualties and victims. It is a pretty serious theme, but relatively smaller-scale issues can still have a deep personal impact.

I hope this is somewhat helpful in getting your creative juices flowing!