Trading button count for depth of effect per button does not add or subtract overall depth available -- it merely makes reaching the keys a bit easier. Such allows for a higher expectation of player responsiveness, but does not demand that the pace of the game is suddenly increased. Nor does it suddenly change the genre to "action combat" (which, by the way, is not a genre).
Neither "tab-target" nor "action" are genres. They're just levels of a particular quality, which then imply certain priorities.
"Action" games require better poll rates, better prediction, appropriate hitbox geometries, and more in-depth physics systems, and tend to make use of these by reducing the button count required in favor of more intuitive and reactive combat, often while maintaining the same overall depth in any given fight (though often also trading depth in personal toolkits for their interactions with party members, enemies, and the environment of play).
"Tab-target" games are what you get when the game can't afford to make appropriate hitbox geometries or more in-depth physics systems. Because the player cannot then make reasonably convenient and precise use of skillshotting when lacking these core qualities, additional keys must be sacrificed specifically for the purpose of targeting particular enemies, though that targeting is still usually less precise or convenient than in any decently-made "action combat" system. In turn, however, this frees up the camera and to a lesser extent the mouse/aim-stick hand, allowing for a slightly different perspective on battle if the game chooses to build around that opportunity. So button depth and player control are sacrificed and, somewhere down the line, replacing these lost capacities grants a couple unique opportunities during static/stationary play (which would be virtually absent in action-combat which has the means and reason to make constant use of the player camera and will thus tend to make it central to its gameplay).
On topic:
Personally, I hated FFXV combat. There's hardly a single element from that game I'd suggest using here, even if only tangentially and with only the very best aspects of its concepts cherry-picked for adapted use. But, I'd agree that our skills (and, each individual combat key) should have more depth. Personally, I think that would best come from making our core skills more modular and each individually compelling, able to be 'combo-ed' in a variety of ways that could together form rotations, but never the same one--or series thereof--over time, given a larger range of interactions with our enemies and environment.
I'd prefer that each skill strike where it appears to, against however many enemies it would appear to. I'd prefer that each does as their animation would appear to do: Vorpal Thrust could push enemies to the side, Full Thrust would either gut the enemy from groin to throat or --if too armored-- 'launch' it into the air, while Fang and Claw could provide an exceptional flanking second strike and Whirling Thrust could disperse enemies as it focuses a core one (alongside a falling vertical cleave). I'd prefer preferential targeting (or, "soft-targeting" if you prefer) over pure tab-targeting. I'd prefer conditional selection, especially for healing, whereby you can pre-queue an ability and select its target at any time before its queue actuates or start casting a spell immediately and select its target at any time before the cast completes, etc., etc. And while I would indeed prefer to see a more active, intuitive, deep, and responsive form of gameplay, those system preferences, at least, I see less as a particular taste in gameplay as simply greater quality in undermechanics and the larger number of possibilities that would be unlocked by those better undermechanics.
If I had to provide a rule of thumb, though -- If the combat system can't keep a player new to a given role but not to the game feeling like they very frequently have new things to learn or improve upon and (mostly intrinsically) compelled to do so over the course of leveling, it probably isn't deep enough. That's the same standard we hold single-player games to. I don't understand why MMOs should suddenly get a pass when they waste obvious opportunities for the gameplay experience over time.