For installing Windows just leave the GPU on the primary x16 slot or the onboard video connected. After the Windows installation is complete, along with all the updates, run DDU to remove the current driver and set Windows to not auto-install the GPU drivers. Then shutdown, connect the rest of the GPU's to the motherboard and install the latest driver. With 12 GPU's it will take a while to install the drivers (20 min.<).
To keep Windows Update from modifying your drivers, disable drivers updates in the Group Policy Editor.
Windows key+R ==> Type gpedit and hit Enter ==> Navigate to: Computer Configuration ==> Administrative Templates ==> Windows Components ==> Windows Update and set 'Do not include drivers with Windows Update' to enabled.
Can I just let my gpu run on psu while I install windows as removing the power cables from gpu is really hard for me.
Also can I install windows on my ssd by putting it in a different hardware (like my desktop) or is it necessary to install windows on ssd while it is in the rig I am gonna use it for ?