Hi Ben,
Thanks for the illustration.
You can create any new Group-Window in Cinema 4D to your liking, but I have no knowledge about saving this Group Window so it can be called up via menu or shortcut, independent from the chosen layout.
Since pretty much any customization tutorial claims that Cinema 4D is freely and totally customizable, I must have a gap in my knowledge, as has the manual obviously; Nor seems any right-mouse-click to unveil some secrets. I even googled it. So, after nearly two hours of renewed research, I have to hand it over.
Having said that: To my knowledge, any user-created window must be saved as part of the layout. If that is free-floating or integrated the GUI as tab (either collapsed or not). Since you can save as many Layouts as you like, this would be all I can share.
Compared to “completely customizable”, my reply here seems very thin. My best advice at this point is, get in contact with the support:
https://www.maxon.net/en-us/support/how-can-we-help/
Besides that, the Tutorial request forum is an idea to voice this wish.
https://www.cineversity.com/forums/viewforum/96/
Fingers crossed.