All-righty.Youre thinking too British, too straight for the HREGN
Nobody would call it the "Hohenzollern domains". Nobody did IOTL, either, even when Brandenburg-Prussia already had incoproated Pommerania, magdeburg, etc etc etc. Or th Landgraviate of Hesse-Kassel - nevermind that it also has included in its territory the Counties of Ziegenhain, Katzenelnbogen and Schaumburg, its still the Landgraviate of Hesse-Kassel, and not the "Domains of the House Hesse-Kassel".
Its ruled by the Margrave of Brandenburg (which is, due to teh electoral title conencted with that, the senior title), hence its the Margraviate of Brandenburg. You can of course then differ between Brandenburg proper and the other territories, but its also the collctive name. Thoug, it would probably be rather called Electorate of Brandenburg - even while its "Margrave of Brandenburg, Elector of the Holy Roman Empire", when short titles were used that was usually contracted to Elector of Brandenburg.
But were the Habsburg domains really reffered to as the Archduchy of Austria?!