Try: improve SQL statement efficiency. I am confident that you will get a 4x-5x performance increase. That's what you need to go from 20 hours->5 hours or less. This won't achieve 15-20 minutes, but that's not what is needed. The idea as I understand it is to make a dramatic improvement in performance with the understanding that the performance won't be optimal.

If you can "make it work" for this abnormal 2-3 year combined update, I would do so. Pursue other performance enhancements if need be (or not).