Yeah it's really easy to get the HRE as France even without diplo/influence because of their large size /proximity to the HRE and especially their +1 dip rep tradition.
That + advisor + max legitimacy + common additonal dip rep missions + papal dip rep = +5 dip rep. Enough to steal from Austria.
After that you can do what the above poster(s) said.
All of this is possible due to the new culture shifting mechanism. The old way, via the "Accept Cultural Shift" decision, was written to limit a bunch of nations from culture shifting, including France, Prussia, HRE, Ming (to any culture besides Chinese ones) etc.
With the removal of this decision, almost any nation can form any other nation with notable exceptions: Papal state, Holy Roman Empire, and tag-locked nations like Qing.
Ming can form Bharat and get permanent claims on all of India. Even better (though more difficult) is forming the Mughals. (Minghals)
These are just some of the new possibilities, I am sure there are other exploits around. I fast-revoked (although not as fast as the GBR shenanigans this version) as Lithuania in 1.15.1 in a similar fashion, though more difficult. Lithuania -> Commonwealth (removes you from the HRE, readd all provinces) -> Ruthenia (removes you from the HRE again, readd all provinces) -> Russia (same, readd all provinces)
This was a much slower way. Might be an alternative to the France strat should it work in the current version.
My one tag using the similar exploit (Lithuania->Commonwealth->Ruthenia->Russia->HRE)