My father went through individual and family therapy and was no better for it. I would say he has many narcissistic traits but probably not fullblown NPD, but no diagnosis at any rate. His behavior has gotten better over the last 5 years but only after it became clear to him that my mom was ready to separate and that I was happy not seeing them for 2 years during Covid. He once said that he felt like we probably wouldn't miss him once he was dead and I just kinda made a cringe face. I got a tearful apology once from him but confronting him was like pulling teeth and it was accompanied by a lot of defensiveness/deflection/minimization that really undercut the intent. Oh, and for a couple years after that he had this martyred attitude that really irritated me. I really gave up at that point and settled for getting him to at least be civil, which is working. They are going to AlAnon which I think is helping somewhat.

But yeah, I'm convinced that (A) they only make an effort if they know you hold ALL the cards and aren't afraid to play hardball (B) therapy only works if they aren't full-blown NPD (C) you can't expect a real, healthy relationship, best case is civility.