With Southwest's most recent devaluation, on an absolute CPP basis, using the URs (and not converting to Southwest) will
almost always provide a better value. You'll give up some of the flexibility of booking with SW points (i.e. ability to rebook and save points, cancel with no penalty whatsoever, etc.) however you can still change a flight on URs, you just end up with travel credits then.
I think your math seems accurate and if you are 100% you're going on this trip and don't need the flexibility of booking through SW RR points, then the URs will be the more lucrative option for you.
All of that said...a side comment: if you already had SW points and were debating between using existing SW points and UR points, then I'd probably go with SW points to use them and keep the URs free for future travel. Hope all of this helps