I can't fully vouch for this setup but there are some dirt cheap HDMI-to-USB devices on Amazon (got mine for $15) now. You can run HDMI out from your main machine to the USB of your iMac and then capture the input with OBS and run it full screen. A little hacky but it should work.
I've been using one to pipe an old Android phones camera to a virtual cam on my laptop.
I've been using one to pipe an old Android phones camera to a virtual cam on my laptop.