Logically you would just put the same dock on both monitors and tell it to move all new windows to DP-2 (or wherever), open an issue on their bug tracker asking for this feature to be reinstated and explaining why, and move on. This wouldn't so much be a hacky workaround as using a built in feature that achieves the same result.

In my opinion primary monitor is conflating 2 settings that perhaps ought to both exist in an easier to use that windows rules.

Open new windows on ____ selected from monitor with mouse cursor, same window as focused application, a selected monitor with the default being same monitor as focused application.

Always show panel in place of any other bar yes/no. Unchecked by default. You can have a panel on each monitor and put things of your choosing in each panel or even have a panel on top and below on the same monitor if you like. So what happens if you have a panel on your primary monitor 1 and also on your secondary monitors 2 and 3? Say you unplug the laptop and monitor 3 becomes the only monitor.

The most obvious and simple thing would be to show the same things on all 3 including showing a taskbar for the windows just on that particular monitor so that when you undock you can still do the same things. Windows does this for example the only difference is which bar has the tray.

The most theoretically correct would seem to be allowing you to set different bars for different configurations but this would be complicated and perhaps little used.

What I've suggested would be to give you the ability to set a primary bar that is always displayed hiding any secondary bars that normally appear on that display.