Is this possible? Iron Man help plz

So I have to fight an average size Commonwealth. I was thinking of:

1. Attack Commonwealth and occupy all provinces

2. Let war exhaustion rise to high levels

3. Take only 1 province with a Lithuanian core

4. Release Lithuanian vassal.

5. Support Lithuanian separatists

6. Declare war again after the rebels spawn (and after the truce

7. Reach 50 warscore to force rebel demands

8. Enjoy my giga Lithuanian vassal that's super happy to be under me with minimal AE and ready to be integrated in ~5 years.

Are there better or faster ways to get such big vassals?

EDIT: So I released Lithuania and Commonwealth still had some nice war exhaustion but then when I went and checked to support the rebels I only found this.

No Lithuanian separatists :(