I thought that is bad solution, all other your qubes will still do not use the GPU, secondary GPU can be used inside the any user qube for improve performance.
For some reasons my i7-8k limited for 2.12Hz, but it’s enough for my daily tasks.
yes, but you can passthrough ony for 1 qube at the same time, using pci features
Seamless GPU passthrough on Qubes OS with VirtualGL. Shortly. You have to hide your GPU pci using kernel params, and use gui to connect your GPU into AppWM.
Not sure about that, above i wrote the reason why not good idea use main GPU inside the dom0. Also try to read this What is the status of iGPU support for Qubes?