The castrate and recruit nerf that makes impossible for the vast majority of characters to do it is, in my opinion, a misguided nerf. Force castration will still let you invite back a character if you have high amenities most of the time anyway. The nerf mainly makes getting eunuchs more inconvenient.
The OP things about making Court Eunuchs are twofold; being able to trade them for huge amounts of influence, and the free Dread you gain from them. Unlike executing, which was nerfed to only give dread against important characters, Castrate will give you Dread for doing it to anyone, making it an OP source of Dread. This Dread income should be removed, only giving Dread under the same conditions executions give Dread.
Second, at 100-200 Influence per Eunuch, turning every male prisoner into a source of influence is OP. This amount should scale a lot more with eunuch quality, and simply only give you the full amount if given to your liege or top liege; with vassals awarding much less influence at 1/10th or so.
I believe these changes would nerf the actually OP aspects of Court Eunuchs (and its cultural variants) instead of the currently palliative, annoying version we have with the things actually making it OP still unsolved.