Honestly, when I think of a sci-fi action adventure without a "endgame" fight.
Star trek, I remember that some some endings weren't a fight l, but just a clever solution. Idk if you can apply that to your story. I think the example was the "war" of the shadows and the vorlons, with humanity stuck between, and even though the two other races were far superior, humanity made strategic moves and won.
I also remember another example from Star trek, (had to look up the thing) but there was a war between two races and one race just showed themselves as billions of years Superior and the younger race just surrendered.
So it's definitely possible. A final battle on equal terms isn't necessary. Seems like other solutions are.
A resolution.
A strategic move.
A one sided win.
A exploited weakness.
You just have to be careful to not fall into the trap of creating a different conclusion for the sake of a conclusion. If you think your work needs a final battle, then by all means, make a final battle. People love final battles on a level field, as endings like the ones above are harder to write and have a higher chance to be a letdown to your readers.
TL;DR - write whatever fits best
I tried to do something like this in one of my short stories, "The Prison of Salvation" might be a good example of something like this. The "creature" wins through a war of attrition instead of a decisive final showdown. Idk if it's good, but it shows that you can still have a good buildup with a decent ending.