Ah, sorry, I got the impression that you wanted the current KDE system unchanged except for the active virtual desktop being per-monitor. So, one monitor has desktop 1/2/3/4, the other has desktop 1/2/3/4, and you can select them independently but they are always stuck on the same monitor. That was pretty much my main thing that I didn’t want. My bad.
The way macOS does is also fine by me (with “Displays have separate Spaces” turned on), where each display has its own list of desktops but you can drag them to other display in Mission Control. I’d just want to be able to do that without using the mouse too 
How it works in sway is, as far as I remember (it’s been a while since I’ve used it), you have a single list of workspaces, each of which are assigned a unique shortcut. Pressing the shortcut to activate a workspace brings it up on the monitor that currently has focus (unless it is currently visible on another monitor in which case it just moves the focus to the other monitor), and monitors have no relation to workspaces except for the one currently active on that monitor (though now that I think about it, maybe it does keep track of which monitor a workspace was last displayed on and shows it there; correct me if I’m wrong). And workspaces are automatically hidden/deleted if there are no windows on them. Slightly different in how you use it but comes around to the same.