The flickers issue still exists in macOS Big Sur
I think GoodGuy12345 is right. This isn’t an issue with the cards or the software, but rather damage to the GPUs caused by overheating. So unfortunately you will need to replace the GPUs to fix the issue. Of course schedule an appointment with the Genius Bar at the Apple store so they can make the evaluation. Make sure you don’t overheat the new ones. Keep them cool by using one of the 3rd party apps to control the fan.