if you want to "dissolve" the HRE per se, then you need at least one empire title aside from the HRE. That way, while the HRE title is your secondary title, you can destroy it.
Still, that still makes the former HRE vassals your vassal, now only pissed-off vassals (who will promptly unite in a faction war of one reason or another).
:laugh:
another method is to hope for a scenario where every single vassal revolts for independence, and the emperor now only has his demesne, wherein he is promptly conquered in various wars BEFORE he dies and an heir is elected among the independent de jure vassals. With the emperor losing his last county, the imperial title goes with it.