Well, high CA elective could represent senates, like in the ERE, so I don't see a problem with that. What I do see a problem with is that the AI is inclined to elect the same family over and over again for the whole 400 years of the game. There's no selfish interest of the vassals currently, they just want what the emperor wants, or what's best for the nation (one of the only times they ever undermined me was to elect my genius, kind, and content daughter).
Elective monarchies should function like republics for the nobility. In the current state of the game, the Doukas family reigns for 400 years. In real life, they only reigned another.. five(?) consecutive years after the game start because the queen gave his seat to a general. Both the HRE and ERE need to at least try to model how they worked in the period, even if that's a little too complicated for what the game can accomplish.
ERE was in no real sense an elective monarchy. A lot of new emperors were usurptions or palace coups (there were a couple of regencies and 1 or 2 end of dynasty adoptions, but they were rare). Regrettably paradox forgot to put this element in LoR, and even gave the Byzantines helpful mechanics to spread the ruling dynasties influence, so after the first 50 or so years, only the Doukas have claims (unless the player gets involved, or the AI decides to usurp with a woman in a regular marriage.) CK2s dodgy fertility mechanics don't help (if every emperor, king and duke has 5 kids (per spouse) and every courtier only has 1 or 0 then ruling dynasties spread like wildfire and disinherited houses simply vanish in a generation or two). As an aside, I'm not sure it even had a senate by 1066 (not that it was more than an infrequent political player when it did exist).
Having vassals get narky if a dynasty holds too many duchies could help the latter (especially in the revoke-with-no-consequence ERE).
But 3 things missings from CK2 that slowed down the HRE.
1) Feudal obligations were for 40 days & furthermore, it wasn't feasible to take castles in a week with a big army and human wave attacks. Or put another way, the HRE shouldn't be able to dump 80,000 troops on everybody - he couldn't use them for prolonged campaigns. One CK compromise would be to increase the opinion penalty for vassal troops. That said you also would need to teach the AI to use (& disband) them more intelligently.
2) In practice, the results of independance revolts in CK2 are frequently trivial. If there is a white peace, the emperor can revoke as he pleases (courtesy of free revoke for revolt/strong claims) if they succeed they will frequently be reabsorbed (due to being an emperor and strong claims). And revokation tends to breed Salians (or at best Germans) everywhere. Solution: the emperor does not get claims unless they succeed, and don't get free revokation on a white peace (its a negotiated settlement that should preclude the revolters being punished after the event.). Also emperors shouldn't be able to pull major difference in rank on dukes. CK having a little more respect for the culture of the province would also help (and the problems that would entail for foreigners trying to rule them- its not the peasantry, its the minor nobility & court).
3) Elective monarchies were invariably bad for crown authority, as the nobles (as a group) worked out they could demand concessions to vote for your successor. I'm also pretty sure they couldn't be persuaded to vote for your distant relatives, over your close relatives. So limit crown authority to low or medium (and of course, reduce the requirement on primogeniture), and cut out distant relatives (if any children or brothers) and daughters (if any sons).