Please present a compelling argument why Hungary should join the HRE when being in a PU with Austria when they never did this while being in a PU with Austria for several centuries.
The game should not necessarily replicate history. But things that also happened historically (Hungary being in a PU with Austria) should not produce ahistorical results (Hungary joining the HRE).
As for the OPMs, my experiences from an Austria game in 1.30.2 happen to be quite a bit different from yours. Several OPMs in Italy (e.g. Bologna, released Verona and Padua) joined the HRE rather quickly, and when I forced France to release Dauphiné and Berry, these two joined shortly before their truce with France expired as well.
I also had the Pope - who joined the empire through the special Imperial incident after I had reined Italy in - of all people expand the empire by adding territories that he had conquered from Naples.
There is no guarantee and no absolute automatism, but it is very much possible to get AI countries to join the HRE and to add provinces to it in 1.30.2. It is ok for this not to be instantaneous and super-easy, considering that in the EU4 timespan there were very few territories that joined the HRE even at times when the Emperor was enormously powerful.
The only historical cases are East Frisia and the counties of Artois and Flanders.