Releasing lots of nations as vassals is one slightly cheesy way to rapidly gain IA, you get a considerable amount by restoring sovereign states within the empire. This does not only include the original states either, meaning that if you have added every province you own to the HRE then any vassal you release will give you the IA boost. This is how I passed all the reforms in about 100 or so years as Commonwealth, and since passing the last reform reincorporates all your vassals automatically there are virtually no consequences for doing this. If you'd like a more legitimate tactic, I think the only other route would be to continue conquering as you already have or starting wars to release HRE minors that have been eaten by other nations.
