Can anyone join the HRE?

Noob here. First playthrough with France. After taking Britanny, parts of Savoy, english mainland on France, and parts of Burgundy, the Burgundy region to be more precise,I wanted to fight Burgundy for the rest of my cores.

Sadly they seem to be a part of the HRE. Although Bohemia, Castille and Aragorn are my allies I doubt I can defeat the whole HRE.

How do I get out if this shit that I am in?

Also Hungary is somewhat part of the HRE.