Wow, amazingly similar experiences. Just to add in, we tried everything. Training collar, calming suppliments, Prozac, many multiple strategies... everything. She was the sweetest dog in her moments, but sadly, incorrigible. We strenuously avoided what we knew had to be done. Still bothers me. Thanks for sharing your experience.
A sad postscript to my experience. The nightly "fight" necessitated carrying our senior poodle upstairs to protect her. My wife would already have the door closed with the "nemesis" inside. I would carry the poodle inside and let her run to safety under the bed. I rather enjoyed carrying her up and she looked forward to the ritual. My wife picked up COVID from a co-worker and brought it home to me. We were both infected before we realized it. The sad postscript is the senior poodle caught it from me as a consequence of the evening ritual. It killed her inside of 3 days. None of the other dogs was affected. On her passing, the evening ritual devolved to just me getting into the closed space without allowing the combatants access to each other.
It sounds like you went the extra mile trying to resolve the problem. Our was strictly a territorial issue. The "nemesis" and "prohibited" pups were best friends anywhere else. It was a heart-wrenching decision.