I'd agree with you- except there have been 2 proper HREmperors in this time frame. Bohemia got a kingdom when Emperor due to (iirc) the concessions in the Golden Bull (HRE constitution almost)- it's not allowed in the law of the HRE that there are more than 3 kingdoms (Italy, Germany, Bohemia). Heck, just to become an Archduchy Austria had to bully/ bribe a lot of the members of the HRE. I think limiting rank to duchy is alright.