You can have multiple monitors and multiple workspaces with multiple windows that can be freely moved between monitors and spaces. The only restriction is that

You can have multiple monitors and multiple workspaces with multiple windows that can be freely moved between monitors and spaces. The only restriction is that when you make a window full screen (not the same as maximizing), it becomes its own new single-occupancy space. You seem to think that making a window fullscreen is the only way to make a new space, but it's just a special case of a larger system that already has the functionality you're asking for.