Yeah this has been made steadily more difficult since the preview build and every subsequent version / patch since.
Used to be you could invite pretty much anyone who liked you to court as there was no 'no reason to move' or 'base reluctance'. Once this was added I started relying on maren....marriter....mariter....reversey marriages to courtiers invited via intrigue decisions, but then they nerfed that with 'unimportant people'.
End of the day, I think the designers want you to make do with the best you've got for a lot of the time, particularly as a count as Cymsdale says. Now I've been forced to have a chancellor with 2 skill on occasions, instead of consistently having 20+, so it ultimately makes the game more interesting and keeps counts from assassinating entire lines of HRE emperors and other silly things.
So best advice is to think in the long term and get women in during regular marriages with the top tier 'education' stat in your required field. Grey Eminence for diplomacy, Midas Touch for Stewardship, I forget the rest. Get them teaching your children, spread education across the stats with various courtier children, and within a generation or two you have a good supply of male and females with good stats who can both be your councillors and teach the next generation.
What I tend to do is, whenever a child needs educating due to the death of their tutor, at that point survey what their highest stat is, and then assign a new guardian based on that stat. Better to have one great stat than a bunch of average ones.
I've been known to swap a child's tutor for this same reason too. At 6 their stats are too low, and lacking any traits, to determine what they will get and what their natural skills are. I know this is influenced by the education but, especially when not educating them yourself, you can't just presume they will get the same general personality traits as their guardian and the yearly non-trait related stat increases are completely random as far as I can tell.
e.g. You have a six year old kid with 2 in diplomacy, 1 or 0 in everything else. You assign them to your main Grey Eminence character for teaching hoping they will become a great diplomat.
They get random stat increases each year, so by the time they are 12 they may have climbed to 7 points in Steward, become Temperate +2 stew, Diligent +1 stew, Just + 2 stew, Greedy -1 dip, and have a hare lip -1 dip, making them 12 in stewardship and 0 diplomacy at 12 years old.
For simplicity let's assume they didn't get any more stats / traits after that point, having them tutored by someone with Grey Eminence, and them getting that trait themselves, would give them a diplomacy of 9, stewardship 12, leaving them competent in several areas but excelling in none. As a ruler this may be preferable, since it helps your state stats across the board. For councillers, however, not so.
So changing a kid with 12 stewardship and 0 diplomacy to be taught by someone who is Midas Touched (even if their own steward stat is not great) will mean that once they hit 16 (again assuming no stat increases, and that they get that education trait) they will have a stewardship of 21. Even getting a less powerful education stat than Midas Touched (which seems from observation to be more likely than having them educated by Midas Touched since 6 years old, since you're switching tutors midway) will mean they are somewhere from above average to very good at stewardship.
Of course, since they continue developing up to 16, which wasn't represented in my example, you can easily get past 21 with a bit of work and luck. I've regularly had characters with stats up to and over 27 this way and certainly in stewardship and diplomacy (which tend to be my focus) I rarely go wanting for good councillors after second generation, even if I'm still a lowly count.
I can only guess at the exact mechanics in the education stat, switching tutors, and AI event choices for traits, but this all seems to work well for me.
Marshall might be the only one I wouldn't care if it was over 20 so much but when you're using him to train troups it's much better when he is.
