First some definitions or my understanding of terms.
JRPG - Story heavy, narrative driven games originating mostly from Japan with anime tropes and featuring turn based character battles commonly
Core gameplay - The main gameplay element of a game, for example Dodging, rolling and spacing combat in Elden Ring, Monster Hunter. Character placement, team builds, XCOM, TRPGs
Sub system - Mini games or the social systems in Persona games, a secondary gameplay element that is different and unrelated to core gameplay
I've enjoyed Yakuza Infinite Wealth, FF7 Rebirth and Persona these few years which led me to think that I enjoyed "JRPGS" so I booted up old "Tales of" game (action battle) and am having a hard time pushing through
I then realised that the JRPGs I've played have a lot of investment into sub systems, Yakuza basically being a collection of minigames polished over several series. FF7 adding even more mini games than the original in a 1/3 installment and also having a non turn based system and Atlus' games having a large chunk of it being a dating sim on top of its flashy UI for the turn based battles.
I think the core reason is that other gameplay mechanics, from driving to shooting to RTS to combat have high skill ceilings such that people playing CS can transition to Overwatch and then to Marvel Rivals and maintain their level of training over years. RTS have high skill floors for PVP. Meanwhile for single player RPGs without deep builds and customisation, you should be able to complete the story and endgame with grinding or items or clever builds. The first 2 being the easiest and least 'fun'
This has led to mainstream JRPGs needing sub systems to support the price tag while CRPGs like Baldurs Gate 3 have builds and branching stories to make it fun and replayable.
On the other hand Dragon Quest 3 hd-2d remake sold like gangbusters so maybe I'm off on this
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.