Frustrations with a 4k 160hz + 1080 60hz Dual Monitor setup, no custom resolution for 1080p panel possible

[Skyy]

I realize my problem might be very niche but I hope I can get some insight before I rip my hair out over this.

My PC with a RTX 4070 runs a dual monitor setup with a 4K 160hz Cooler Master GP27U and an old ASUS 1080p panel, both run over Display Port. Despite the bugs of the GP27U's firmware I love this monitor for its picture and generally have next to no issues with it during regular usage.

There are a few problems however in a multi monitor setup. For the first problem, my Nintendo Switch is also connected to the GP27U. For whatever reason the headphone jack would only route audio from the HDMI connection of the Switch if I turn the monitor off and then back on while it's on the HDMI input. That's really not a big deal but thanks to the ever so annoying Hotplug Detection of DisplayPort chaos ensures once I turn the monitor off.

The GP27U runs at 150% scaling and the ASUS at 100% scaling, nevermind the different resolutions. So as soon as the GP27U turns off everything displayed on it wanders off to the ASUS monitor and because of the different scaling factors too every window and program gets resized and often the entire UI of an application becomes a mess even if the programs move back to the GP27U when it's back on. Nevermind that dragging certain applications between 150% and 100% scaling monitors sometimes results in very weird effects.

I've been hit with an epiphany though, what if the ASUS monitor also runs at 4k with 150% scaling, then everything would be uniform with the added benefit that dragging anything between these monitors is now seamless! Should be easy enough right? Wrong!

My first try was to setup a custom 4k 60hz resolution for the ASUS monitor in the Nvidia Control Panel. That worked like a charm, everything worked like I hoped it would except I forget that, as soon as you enable Nvidia's custom resolution support even just for a secondary monitor you lose G-Sync on your primary monitor. As soon as I disabled the custom resolution G-Sync sprang back into action.

I made a post on the Nvidia Subreddit about it which got replied with the suggestion of just using DSR before my post was deleted. However I'm unable to tun on DSR since you can't just enable it for one monitor so I'm completely locked out of DSR since the GPU27U uses DSC and even if I drop to 4k 120hz or 4k 60hz as a test I get no DSR options what so ever.

As a last resort I wanted to simply add a 4k resolution to the ASUS monitor by using CRU. I added a new Detailed Resolution copying the exact same values the Nvidia Control Panel gave me when setting up a Custom Resolution over there. However no matter how many times I ran restart64.exe or rebooted my PC the 4k resolution would never show for the ASUS monitor, the highest is still "1920x1080 (Recommended)"

Is there any way I can run my secondary screen at a downsampled 4k resolution without losing G-Sync on my primary screen?

Thanks for any discussion pointers in advance!