Glenwing

Well, this doesn't really solve the issue, but HDMI 2.0 does not support 4K 60 Hz 10 bpc (unless chroma subsampling is used).

tftcentral.co.uk

I don't really know anything about the Valve Index. If it's using the DisplayPort MST protocol to send two separate display streams though, it won't work, since this protocol isn't supported by HDMI, even if the combined resolution is within its limits.

If that's the github version of that guide, it's been a long time since that's been updated the one pinned to the top of this forum section is more actively maintained. Still, it doesn't include any adapter that suits your needs, as that one I haven't updated with recent developments either. You can use this adapter for HDMI 2.0 source to DP 1.2 display conversion: https://www.amazon.com/gp/product/B08FCHQXSQ/ I've tested it up to 2560 × 1440 @ 144 Hz.

Generally games have a fixed vertical field of view and then expands horizontally, so what you can see in game depends only on the aspect ratio. But this just depends on the game, some games such as Civilization will just let you see more things if the screen is higher resolution.

Moved to General Discussion, as this is a discussion topic, not a tech news article.

https://www.reddit.com/r/Monitors/comments/egs2hn/ultra_hd_hdsd_nvidia_control_panel_capped_at_60_hz/

You need to plug in a secondary display and use that one to change the settings.

That's the edge of the display.

Please check the pinned thread on cables and adapters to determine what you need for your situation. For DP to HDMI:

Do you meet all the requirements? https://linustechtips.com/topic/1021857-g-sync-compatible-master-thread/

Make sure the output dynamic range is set to full.

Use borderless window mode, not fullscreen.

Have you set the scaling mode in the NVIDIA control to Display?

Probably just the way the monitor is calibrated. Have you tried different settings on the monitor, or just been using the defaults?