The answer is : the elector mechanics are not designed very well. That goes for a large part of the HRE mechanics too.
First of all, all AI electors are hardcoded to immediately vote for themselves if they have at least 200 dev and are eligible to do so. They do not care if there is a 2k dev ottomans next door that they will get destroyed trying to defend the empire against, and the electors do not care if there is a 2k dev nation in the HRE that would be better suited to defending the empire. They will rate a 200 dev nation and 2k dev nation equally as "large nation in the empire". This is also why sometimes you see a 200 dev nation become emperor and then get destroyed by ottomans/PLC while answering sucidal calls to defend the empire.
This is not moddable btw. Like most of the HRE mechanics.
Secondly, an AI emperor will refuse to make anyone an elector if they have more than a few provinces. I think its more than 3 or something stupid. They are afraid you will be eligible for the emperor. The AI does not care if you are a theocracy or republic, and hence, ineligible to be emperor anyway.
Thirdly, unlike the emperor election, there is no logical mechanic to obtaining an elector seat other than PUs, which are usually extremely random. If you did the most of the heavy lifting during the league emperor and the new emperor loves you? Too bad, they still wont make you elector because you have more than a few provinces. Its a catch-22 : if you only have a few provinces to be eligible, you are too weak, but if you do have more than a few provinces, the AI wont make you elector anyway.
Many other issues with the HRE system that were never fixed with the emperor DLC :
-Your own subjects will refuse to support reforms and can refuse to vote for you as the emperor. This obviously makes no sense. Large PU subjects in the HRE will never support your own reforms.
-You can no longer mouse over a reform and see which princes are not supporting it. It used to show it in 1.29, now you have to manually mouse over -every single prince- to see who is supporting a reform.
-The AI is hardcoded to be neutral towards the rest of the HRE if they are not rivals. So Bohemia will instantly stop caring about taking Austrian provinces if Austria becomes too weak to be a rival, and this breaks their mission tree. Bohemia will never use the subjugation CBs on saxony/brandenburg either as they are too weak to be a rival.
-There is no easy way to see who the heretic princes in the empire are. The religious mapview is not an easy way when there are 50+ princes in there.
-The AI emperor doesnt know how to enforce religious unity or take back imperial territory, so imperial authority drops to 0 and remains there for the rest of the game.
-An AI league leader has no idea how to seperate peace out heretic princes to force convert them to avoid the massive loss of authority after the league war is over.
-Ironically, finding some flimsy excuse (fabricate claims) to declare on a heretic prince is a better way of enforcing religious unity than the diplomatic action, as the diplomatic action gets you a stacking -20 opinion penalty with every heretic prince in the empire. And it decays very slowly.
-Free cities will go and get themselves killed by losing wars (that they started, hence, the emperor cant intervene) or by anwering sucidal call to arms. And you cannot create new free cities easily without client states, nor can you force someone to release an OPM as a republic since they will always be released as the original government. So it quickly becomes impossible to replace the free cities after a while, till you get client states or stack enough dip rep that you can convince monarchies to become a free city.
-Decentralisation path is obviously way worse than the centralisation path in every way. You only take decentralisation if everyone agreed on it in a MP game or you want to make a HRE game harder in single player.
-The "subjects in the HRE do not take up dip slots" thing is, strangely, hardcoded to the hereditary HRE reform, which makes absolutely no sense. It should be tied to the revoke.
-AI nations will almost never join the HRE, doesnt matter if you are sitting at 100 IA and they are threatened by ottomans or whoever. Use "mapmode aihre" in the console to see for yourself. Its broken.
-Nations that you release do not join the HRE, so you have to wait out the truce then declare on them with the expand empire CB after. This makes no sense.
-Your own subjects do not join the HRE, except for an event that lets you petition the emperor to let your PU join the HRE. For some reason, the emperor is not allowed to do so, so the emperor's PUs can never join the HRE.
-The "sided/went against us in the diet" opinion modifier NEVER expires, even if the HRE has been dismantled. This is obviously a terrible idea.
-Getting enough electors in the same war to dismantle the empire is nearly impossible as there will always be at least one that you cannot ally, and are not allied to the emperor. This forces you to do silly workarounds like seperate peacing the emperor and then forcing the emperor to answer sucidal call to arms to get enough electors into the same war to occupy their capitals and dismantle the empire. A "dismantle the empire" CB would solve this problem.
This is not even an exhaustive list of HRE issues....
