I finally got around to setting up Linux, Kubuntu, on an old Mac mini. And today I got Barrier, a software KVM switch (although it is really just a KM switch) running (there is an extra step needed with Monterey - Stack Overflow to the rescue!).
Anyway, Barrier enables a shared clipboard. And I was able to copy/paste from macOS to Kubuntu and back again.
I think I’ve found the answer!
KDE Connect works between all monitor platforms and mobile devices. So far I’ve only used the shared clipboard function, but there is lots more to explore. https://kdeconnect.kde.org/