2012 iMac not recognizing external monitors

I have a 2012 iMac, 27" OS Catalina 10.15.7 (yes I know it's old). Up until two weeks ago, it was the working machine in my recording studio, supporting 2 external monitors, running for the last 2 years with no problems at all. Suddenly one morning, the external monitors are not working. I read that 30hz cables were the culprit and 60hz were the ones to use. Mine were 30hz, so I bought new 60hz - no change. I tried every reset, restore, different cables, 3 different monitors and a HDMI TV....got nothing on any of them, but the studio monitors work fine with my 2019 Macbook Pro. The iMac isn't recognizing them. So after a lot of research I found others who's monitors had quit after updating to 10.15.7 so I assumed that I had been lucky for mine to work all this time and now whatever glitch the other folks had got, I now had it too.

So, I did a full Time Machine back up, and started over with Mountain Lion, in an effort to find out 'where' the monitor problem happened. So, Mountain Lion installed, still not recognizing the monitors even when 'forced'. So upgraded to El Capitan....same, no monitors, and can't get the Mac to recognize them, tried deleting a file in the Library according to another tech post, but it didn't change anything. So, I upgraded to High Sierra, again checked and double checked every cable, PRAM/NVRAM. I think I've read about every post concerning monitors, over the past 2 weeks. But, alas, still no monitors. Again, they work fine with the 2019 Macbook Pro, but nothing works with the iMac, it simply will not recognize a monitor, or TV. I'm at my wits end! My logic says it's a hardware malfunction, but I thought it was worth putting out there to see if anyone has any ideas, please?

Thanks so much for reading. Any help greatly appreciated.