I think there are too many JRPGs that still use their battle system in support of their narrative for it to be considered anything other than a core system in those games. That’s especially the case in lower budget games in the genre.

Larger budget projects are branching more and more into side content/worldbuilding, but I’d argue it’s still highly underdeveloped in the genre when compared with western RPGs, in quality if not also in quantity. Persona and Yakuza are exceptions, rather than the rule. Persona is doing something entirely different (and well enough that it’s being emulated now) while Yakuza, as you say, carry a lot of that over from prior development into its RPGs from the series’ action games.