Working on an old bike with a hodgepodge of groupset components - 5500 "105 flight deck" shifters (apparently 9 speed from what I've been able to work out online), a 3x Sora crankset, and a 9 speed cassette in the rear. The rear derailleur was an old Ultegra whose outer cage plate literally snapped off while cranking on a particularly steep incline (the thing was literally caked in the nastiest grime I've ever seen on a bike for years - not my doing, just started riding it a month ago). I would ideally like to replace the derailleur with an unused component but am having a hard time figuring out compatibility with this system - I can't find any rear derailleur corresponding to the 5500 series via google searches and the more modern 105 options are all either 10 or 11 speed. I'm new to the bike repair game and so far have come to understand that my main restriction is the number of speeds has to match between the shifters and the derailleur, but I've also seen some sources say that the cable tension standards for groupsets can change in big generational overhauls.
Ultimately, I'm first and foremost looking for any suggestions regarding compatible derailleurs for this bike as well as some pointers on what to look out for when considering component compatibility in the future.