This seems to be a KVM switch, not a boundless/roaming KM switch?
Note no "V"; it's to have the mouse seamlessly move from one desktop to the other when the machines are each connected to their own screen. Browser-based is also a non-starter for that use.
I'll definitely have a look if/when I need a KVM over IP switch, though.
https://github.com/tiny-pilot/tinypilot
It's pretty easy to extend since it's just Python and vanilla HTML/JS/CSS, so there's not much of a toolchain to install.
You can buy it pre-made for $399 or make a DIY one for under $100.
https://tinypilotkvm.com/blog/build-a-kvm-over-ip-under-100