I think I finally solved the problem with the following method: The external display on my Mac mini 2020 (M1 Mac) was flickering, so I switched the OS and switched the HDMI port to a different one, but no improvement. However, one day, I thought of something and switched to a thick HDMI cable that I use to connect other devices (4K TV), and it became stable like a lie. Did switching to a thicker cable help? The thicker cable that worked so well didn't have any special cable specifications on it, it just said "High Definition Multimedia Cable". I also tried another experiment. I also found a very thin and flexible type of HDMI cable, so I switched to that. This one was also stable and displayed well. What does this mean? In other words, I would have preferred a thicker, more solid cable, but the thin one displayed fine. I think a good quality cable will solve the problem. You should all try it.