I'm still playing FM21, but I also find it very hard to change some personalities. Sometimes it changes, but other times it doesn't. I guess you also need to have a bit of luck. It is what it is.
Unfortunately I had to say no to young talents who could potentially fit the way I want to play because they had personalities like fickle or mercenary, which I find incredibly hard to change. I'm even a bit skeptical when they have a balanced personality, because I hate that one too lol.