Sunshine is actually pretty easy to set up. Just install it on the PC, and connect from a Moonlight client on the same LAN. The complicated part is if you want to get fancy with the networking, for example if you want to access it securely from outside the home, or if you run Sunshine inside of a VM and want to access it from outside the host. But if your laptop can handle the games you want to play, turning it into a game streaming server should only take an hour tops. Definitely easier than messing with passthrough and virtualization.

KVM switch is the easiest way, you just plug both GPUs into your monitor, and switch between the two depending on if you want to see your main system or your VM. The Archlinux wiki also talks about "Looking Glass" which passes through the VM's display to the main system. I haven't tried this so YMMV. Alternatively you can run Sunshine inside the VM and stream to the main system. Maybe overkill if you are only using a single PC, but if you've got other devices you can stream to those too.

As a veteran to VFIO and GPU passthrough, I would recommend the KVM option first. It's often worth buying cheap workarounds to start with, before diving into complicated software and networking setups like Looking Glass or Sunshine.

Intermediate route: pass through your GPU to a virtual machine running Windows or Linux+Proton. This is the strongest isolation aside from dual boot or using a second device, and runs almost as well as native. There are a lot of tutorials online but the archlinux wiki is a good place to start. This usually means you need a second GPU for your main system (an iGPU works if your CPU has it), or you can use janky scripts to switch the GPU between your main system and the VM. You also might need a KVM switch to switch your monitor and keyboard between your main system and the VM.

Expensive route: if you have two PCs you can set up one for game streaming using Sunshine, and stream games to your primary PC. Benefit of this approach is you can also stream to your tablet or android TV.