It sounds like what you really need is a mesh VPN not really KVM.
Install Tailscale on all the machines and you're set.
Alternatively Hamachi.
Edit: You could also install Tailscale on a single dedicated device on your destination network (such as a Raspberry Pi or mini PC) and configure it as a Tailscale router. This would enable you to access any IP-based device on the network without that device having Tailscale installed on it.
With any of this you could access machines just like on the local network, using VNC or RDP.