https://www.fanfiction.net/s/1213270/1/A-Promise-Redeemed — Ranma loses Akane, ends up in the medical profession, becomes an international first responder. Ran+Nab.
Yep, took me all this time to find it again. I only found it because I was rooting about in my local copy of the Nabiki and Ranma Relationship Archive looking for something to read.