Have you tried grouping your arch sections?
That took care of the slight difference in lightmap colouring of the sections, which will have the bonus of also smoothing out the normals.
(Ignore the random texture and the little speck of light in the lower right part of the image, there’s some missing stuff that’s leaving the house above open to light from the directional light and the big block on the right isn’t set to twosided because it’s a solid block, so it’s reflecting against the lower inside of the arch it seems)