> An alternative could be a good 5K/6K monitor with a KVM, but this is a unicorn as well...

I use a Dell U3224KB (32" 6K) in this way. Thunderbolt (usbc) is connected to a Macbook Pro and DP & regular usb c is connected to Windows gaming machine. Works perfectly. And yes, the U3224KB is pretty ugly, but I've grown to love it's features.