> I have thought about a HDMI/keyboard switch to allow me more easily switch between my different spare computers into the left “backup screen” to make it a lesser hassle to alter between those machines, but I haven’t yet decided nor found a product that would be suitable.

Personally I love the Level 1 KVMs but they only do DisplayPort now.

https://store.level1techs.com/search?q=kvm

I have this 2 monitor 2 PC one https://store.level1techs.com/products/14-kvm-switch-dual-mo...

Still the KVM market is “all over the space” both in available configs and actual build quality too.

It would be extremely niche but an open source modular KVM is a dream of mine.