UPDATE:
in the meantime, i upgraded my 2010 27” iMac with a Nvidia GTX780M and successfully got Catalina to work on it (not easy, many sleepless nights, but well worth it!). Worked seemlessly for months!
Now - only 8 months later - it looks like that 780M is broken. Computer crashing, and back in Chime Loop I am :-(
Probably going to try and bake this card as well. Any other suggestions?