Hundreds of attempts later, after 10+ dupes of 504, and countless 503 pace runs that got the elevator softlock (if you hit the elevator button on the first possible frame, it softlocks), I finally got the 503. No failed sprint cancels, no major stucks, the only time that can be saved is with even cleaner sprint canceling, or if something new is discovered. This is one of my best runs in a long time.