I agree the Jurchens need a unique government, like everyone else is saying; the horde penalty just cripples them. Before that was changed back in Cossacks, they used to actually conquer Korea pretty regularly.
With the tributary system and all the other changes in the past couple of years I wouldn't be surprised if some other stuff will still lead to their conquest more than it should, but until the horde penalty isn't a factor for them I don't think any other change would make any difference.
How do you model a situation where a 5-6 million people nation regularly gets slapped around by a 200k people nation?
Korea had 10+ million people for most of EU4's time period. I suppose that just reinforces your point more though.
I do think it's worth stating that the Koreans conquering Manchuria isn't really as horrific a turn against history as people often make it out to be here though; especially in 1444 the Joseon kingdom was not nearly as militarily inept as it'd later be, and Korea'd had plenty of successful conflicts against various northern neighbors over the centuries, including some against Jurchens just before the game's start, where Korea conquered its northern provinces. A bit different from conquering all the way to the Amur River, but by EU4 logic, it kind of makes sense. Still very bad that it happens literally every game though.
Also, it couldn't even smelt iron until Nurhachi and should've imported ironworks from China and Korea.
Have you got a source for this one? Knowledge of ironworking came to the Korean peninsula
from Manchuria back in the 1st millennium BC. It's a bit different from the high quality steels that were getting made by EU4's time, but I'm skeptical that the Jurchens were incapable of making iron, even if they mostly chose to import.
Also I've yet to see a reliable figure for the population of Manchuria in this period; it was (almost) definitely markedly lower than Korea's, but especially since development is pretty loose in how it's determined anyway, and the Manchus were historically so significant, the development discrepancy makes a bit of sense.