That does not work from the dock, but after doing some research I think I found a third party app that does this essential function for Windows users, Dock Mate (apparently it's a replacement for and older app called Hyperdock).Ah, that is a different scenario. Those apps are more for tiling windows whether split or multiple.
Regarding your multiple windows of one app, are you using cmd-` to toggle between an app’s windows? I think you get previews that way. I don’t recall as I rarely need that but that what I vaguely remember. It is the intra app version of cmd-tab to switch apps.
It's paid and it's a shame it's not built into MacOS but at least it's available...
Now another problem I am try to solve is the poorer management of external displays by MacOS compared to Windows. For instance the fact that you cannot have a dock on each screen. My research shows that it's impossible, unless you use a replacement dock like ActiveDock, but I don't want to replace the MacOS dock, I just want 2 or more, one on each monitor... sad...