Wow, CKIII sucks for modding? I thought it was even better, has so many interesting features for modding potential. Weird.
I haven't looked deep enough CKIII to know.
CK2 had a lot of script functionality added throughout its life cycle, but CK3 is closer to CK2 at release without all of this. To give a couple examples, in CK2 as of patch 2.8 you could make a decision that has a "third party filter" which means a targeted decision can also require a second target (who is the third party), but from what I recall this functionality is not available in CK3 at all. As of patch 3.0 that came with Holy Fury, you could further apply "scripted score values" to these third party targets in order to assign them arbitrary values to determine how good they are for the decision, based on e.g traits and abilities of a character (or whatever you can think of). Scripted score values do not exist in CK3.
As a more practical example, since it may not be obvious what you'd actually be able to do with these two features, third party filters and scripted score values allow for things like creating a slavery system where slaves have prices based on their abilities and can be sold for a price determined by a scripted score, or even traded between characters. In CK3 all of this is impossible, because you can't have a targeted decision with a third party (i.e. you can't target a character, pick a decision, then the decision lets you pick another character) and there is no way to apply scripted score values to characters.
Of course, such simulationism is unfortunately a bit pointless in CK3 in any case due to how it treats characters in general, since courtiers are unimportant and purged nonstop from AI courts for performance, and AI character traits don't matter since events constantly change them as they fancy rather than base what happens off of them.