still overpower in any human's hands. I can't see how poland can not receive another round of nerf.
Meh, Denmark/Greece could do the very same thing much more effectively even pre-nerf with enough fighting, same with some others. Now those civs will just do the same things better, but with some effort needed in exchange. Same with Ramesses/Kamehameha. I don't find Poland a problem, there's much better civs now and there were better ones even pre-nerf. If Byzantium gets +1C per 2 followers as it's bonus belief, it can expect more policies than Poland too. In fact, almost anyone with +C anywhere can compare.
I've been thinking about Poland and Casimir (from a historical perspective).
Currently, the UA is immensely powerful and available from the start, whereas the UB and UU come quite late. Considering the UB is a replacement for the stables, it's use is quite limited and underwhelming vs. other UBs, because stables are rarely built, even though the UB is an great improvement vs. the stable. And the UU imho could come sooner and replacing the knight, considering Casimir reigned in the 1300s.
So my suggestion would be to nerf the UA and buff the UU and UB.
I'd rework the UA so it would reflect that Casimir was a great diplomat and strategist (military- and politics-wise) who greatly expanded Poland's territories and influences, that he ensured stability between the competing factions/classes within the Polish society, was considered a king of the people/poor, implemented many then-modern laws, protected the Jewish population, etc. So perhaps the UA could be a combination of 1.) less unhappiness not just from crime, but also all others (religious division - protected Jews; poverty - king of the poor; illiteracy - modern laws, created Academy of Krakow), 2.) diplomatic/political prowess (not sure how) and perhaps even 3.) military prowess (not sure how)
I'd change the UU so it would be a knight replacement.
I'd change the UB so it would be available in the medieval era of one of the buildings more commonly built&generally useful, not something specific like the stable. Considering he built over 50 castles, it could be a castle replacement. Considering he found the Academy of Krakow, it could be a university replacement or "Oxford's university" replacement. Or sth else.
I'm sorry, but your suggestion is not too good. Why introduce yet another Knight replacement when Hussar just fits way more in Renaissance when unique Lancers aren't a common sight? Sure, WHussar is a bad UU because it's unique trait is more annoying than it is useful, but Renaissance is where he should be. UB I could see changed, but Royal Stable works fine as is and I don't see the point.
I can think of something better.
From wood to stone/brick /whatever name
Units can't be bought but only invested in. Invested units cost no maintenance and start with a random accessible promotion. Adopting a SoPOL or starting a GA invests in all cities.
Fits Poland as the first part reflects the time before Poland's fall when nobility was too greedy to buy or pay units (so they only pay a bit) which would weaken Poland defensively, but what the army was was still good as it was true patriots. Investing in cities to build stuff faster fits Casimir as he "has found Poland wooden but left it made of brick". Very unique, no one else can only invest in units. Investing units of course should only cost 50%, but it's still bad because you can't instantly count on them if stuff happens and you need them, though they're stronger because a Horsie might get free Shock or Drill. Fitting.
UB - same, but +2 Culture because the UA is not as good. No +supply% unlike Stables (unless the supply change from current patch is removed) to further show nobility not really wanting to give anything for an army, instead +10 GAP points when mounted/armoured units kill.
UU - same, but at least 40 CS and the super charge actually deals more damage if someone can't retreat, I think that part either doesn't work or is so weak I can't feel it deal more damage
UU could get the buff regardless if any UA changes are made, it's a pretty bad one. Maybe among the worst.