There is yet another strategy to deal with getting the ribs sanded to match. Don't have two ribs.
For a number of wings I built, I produced a thicker rib for that location. It seemed to me that all you actually need at that rib location is a thicker surface area for the covering so it won't pull away in the depression it sees. There is no real value to having two halves glued together for strength. You can't beat the strength of one piece. What matters is getting the spars, LE, and TE connected strongly. Where that rib (or two) mate to the LE, TE, spars matters in that it helps support the butt joints, and having just one wider support is often easier to match up.
The wing sections got built with the thick rib pinned in to insure a more uniform wing, then pulled out and used to glue up the other part of the wing.
I use a right angle when framing both halves. Leave some extra length to the LE, TE, spars and when joining the polyhedral break, sand to fit. The right angle insures the halves join with no front or back "sweep" to the outer part. Paper plans will sometimes be "tilted" or have a "lean" to the print. It's always worthwhile to check for that with a big right angle. You still will need to bring the two halves of the wing together with a sanding block. Getting 4 points to match up perfectly isn't going to happen on the 1st fit. Having an extra thick rib to back each butt joint has always seemed stronger to me. And easier to do.
If you make the thick rib just twice as thick as one of the originals you are discarding, you also wind up with a lighter wing. Heck, you may save a thousanth of an ounce in glue.
That ought to result in at least an extra minute of time on your next thermal......