I spent some time unpicking and re-sewing this afternoon, and this is the result:
I'm much happier with this arrangement. Even though it's not how the original quilt is put together, I really like how the narrow inner border defines the edge of the pieced blocks. I also like how it tamed the problem of all the bumpy seam intersections between the main body of the quilt and the zig-zags. There are only two further zig-zag sides to put on, and then it's the final wide border, which is the same fabric as the sashing strips and the narrow inner border.
So happy with how this is progressing, I'm really on the home stretch :-)