Afaik the X11 standard says that this shouldn't be done and that workspaces should span all monitors (or something along the lines of that), thus most DEs don't do this (I've read this in the gnome issue tracker), don't ask me why cause I also hate this behaviour. Most window managers will do that however and luckily it's super easy to replace xfwm with another window manager. I use i3 inside xfce on my work laptop, this guide describes how to set it up with ease

Isn't the whole workspace thing simply a software way to have multiple monitors into one which you can "look at" when you switch?

In i3wm/swaywm, hyprland, ... You can assign workspaces to outputs (monitors)

Do you mean you want separate sets of workspaces on each monitor and to be able to switch through them independently? Just having "workspace 1 on monitor 1 and workspace 2 on monitor 2" sounds no different than the default behaviour with no extra workspaces.

SwayWM, but I think any of those you mentioned should be able to do it?

I think maybe Enlightenment can do that, but I'm not sure.

Normally workspace definitions are systemwide not monitor specific. A workspace uses all monitors on the system.

AwesomeWM should allow switching workspaces independently if you want to try a dynamic tiler

What's the workspace give you? A bare task bar?

Gnome 3 has an option to keep one display fixed when changing workspaces... Also most window managers allow you to keep certain windows on all workspaces, maybe that will help?

KISS: Plug workstation 1 into monitor 1 and workstation 2 into monitor 2. Then use something like Synergy to share the keyboard and mouse between the computers.