I have had great success with a generic KVM I found on amzn [0], but it now appears to be unavailable, which is a shame because I wanted to buy another. Perhaps I can find it somewhere else. It has never failed to switch between sources and the only delay, as some others are pointing out, is the HDMI handshake process inherent to the monitors themselves.
I did have a considerably more expensive TESmart [1] but couldn't use it because it had some weird incompatibility between it, thunderbolt docking station [2] I used with it (which also work flawlessly), and the MBPs connected to it. I could never figure out why, even raising the issue to support with no luck.
My two cents.
[0] https://www.amazon.com/gp/product/B088WGKFZH