Thank you @apparatus and sorry for the slow reply.
I decided to wait for the kernel fix and now the boot process does not get stuck anymore when using my NVIDIA GPU! 
I am back at trying to disable it in dom0 to pass through it to a dedicated VM.
My Nvidia has pci id 02_00.0.
Hence I have added the option
rd.qubes.hide_pci=02:00.0
to grub.
This works, in the sense that as soon as Qubes boots, the monitor goes off which means that the NVidia card has been correctly ignored.
My issue, though, is that I don’t get a video signal from the other (integrated) Intel GPU.
This is the case even if I power on my machine with the HDMI cable connected uniquely to the Intel GPU or not.In this guide it seems to suggest that I need to configure the BIOS to declare a correct order? I can’t find the relevant option (I have a HP machine)
Are there other solutions, specific to Qubes, to use a specific GPU?
Thank you very much again
Edit! I finally found the right option in the BIOS!
And now Qubes boots flawlessly using the Intel (ingrated) GPU and ignores the NVIDIA GPU.
To verify that the Nvidia GPU is indeed properly ignored, I have executed (as indicated in this guide ) the following command:
lspci -vvn
and my NVIDIA GPU has: Kernel driver in use: pciback.
I can finally try to complete my GPU passthrough configuration! 
Thank you again @apparatus !