That already looks filled and finished for the most part so taking it off would be a bit of a chore.

You could dry fit a couple small filler pieces to hide the gap. One straight across, one on an angle back toward the vertical. Pick one you like and fill it I as best you can?

At the end of the project you might not see it that much once it's all finished. You notice more of it now cause you just noticed it. Hard to say from just a picture