I have an Asus RTX 3060 OC 12GB, with same problem. I've tried everything you mentioned without success.
I have a Samsung Neo G9, so it would be nice to aquire 240Hz instead of 120Hz. I use it as a dual display: 1 HDMI from the 3060 and one cable via the integrated mobo card. It's working, but it's not the real solution.
Is there any real solution?