Those bars and their adjustment is not my favorite setup. At 76, I've come tp prefer a compact bar set up like this, note the vertical brake levers:
https://www.bikeforums.net/19145009-post3.html
I ride about the same drop as you show. I think changing your bars and their setup will have a larger effect on your comfort than changing the stem. Right now, I'm running these bars:
https://www.competitivecyclist.com/f...pact-handlebar
They have the modern and stiffer 31mm center section. Looks like your current stem might be 27mm, in which case you'd need a new stem anyway.
That's a beautiful classic carbon bike. I ride a '99 5200, purchased new. I never saw a reason to buy another single bike. My only quibble with the 5500 is that they came new with a cassette which was too small for most riders. Back then, the more you paid for a bike, the smaller the stock cassette.
I confess to be bothered about that perfectly normal road bike being a "living hell to ride." If that's the case, you'll not be looking forward to a very comfortable aging process. Whatever's wrong with you needs physical therapy. The bike does not need therapy beyond a new bar setup. If you've had spinal fusion or similar, I take it all back. Change out the bars anyway.