Linux

I did it a few years ago, although I had an AMD GPU.

It works surprisingly well but games that require anti-cheat software (ex. Valorant) won’t work.

The only other good solution that wasn’t mentioned in your post is to just buy and maintain a dedicated windows box. Short of that, GPU passthrough is a great option

I currently have a VFIO setup and it works great. There's just almost nothing I need it for with how great Proton has become. I have an AMD APU and GPU. My Linux desktop runs on the APU and only offloads to the discrete GPU when invoked with the DRI_PRIME=1 environment variable.

Virt-Manager has the ability to run scripts at certain points in the VM startup and shutdown process. This is what I use to reassign the GPU to the VFIO stub driver so it can be handed to Windows, remove half of my CPU cores from the process scheduler so Windows isn't suffering constant cache misses, and open looking-glass which uses a shared memory device to render the GPU output in a window on my desktop with minimal latency. Scream starts on login to handle audio, and you're going to want to use a shared memory device for that because it has latency problems over network.

I've been told I have one foot in each bucket labeled "single GPU passthrough" and the other labeled "dual GPU passthrough". If you've only got one you'll have to exit your X or Wayland session to use the VM because your GPU cannot abide two masters. Nvidia has a functional equivalent to Prime, I think, but I don't think it just works out of box. I understand Nvidia isn't happy about their consumer cards being handed to VMs as it's usually an expensive enterprise trick, so there might be a workaround process there.

The Arch wiki has a great tutorial on GPU passthrough using VFIO and OVMF, that's probably your best bet on an arch based system like Manjaro.

I'm not going to watch the video, but what's the procedure for switching between Linux and Windows? Usually you dedicate a GPU entirely to VFIO, with a 2nd GPU for the host OS (or run headless).

Anyway, will it work? Yes, minus some anti-cheat software. Will it be a simple solution? Well, once you get things stable, yes. The tech behind this is mature, but it can be a rabbit hole.

I would look into a non-Nvidia GPU for your 2nd PCIe x16 slot (x4, shared with the 2nd M.2 slot FYI). Good idea to check IOMMU groups before buying anything, but modern AMD motherboards are usually fine. Blacklist the Nvidia drivers and dedicate the 3070 to VFIO to make your life easier, and run Linux off the secondary GPU. Intel A380 might be a good choice. Do gaming stuff on Windows and stream via Parsec/Looking Glass/Moonlight+Sunshine; everything else on Linux.

i am writing this message from a single gpu vfio setup

I do this for testing graphics code on different OS/GPU combos - I have an AMD and Nvidia GPU (hoping to add an Intel one eventually) which can each be passed through to Windows or Linux VMs as needed. It works like a charm, with the only minor issue being that I have to use separate monitors for each because I can't seem to figure out how to get the GPU output to be forwarded to the virt-manager virtual console window.

I have almost the same setup hardware wise. I got an RX550 GPU for the host, or sub in any cheap graphics card. If you have the spare slot and lanes its much nicer of an experience to have 2 GPUs. Plug a dummy plug into the guest GPU with looking glass set up and its great! :)

I now have two gpus, but I've run single gpu passthrough for a long time without any issues. However, you have to keep in mind that some software (such as anticheat for games) will refuse to run in a virtual machine.

I used to use VFIO several years ago, but these days every game I want to play/ have played recently have all just worked out of the box with Proton. VFIO was always fragile, but it worked great when it did. It has the same problems Proton has with anti-cheat, since its still a VM. So your milage varies depending on the games you want to run.