Can't say I've ever gotten much challenging gameplay out of the series myself if I'm being honest with myself. Even the best of the games have rather trivial combat systems when you break them down like that. You could make a similar argument for Final Fantasy V: it's entirely possible AND practical to beat the entire game using nothing but the Wind Crystal jobs. The occasional new weapon might tempt you to try a Dragoon or something, but most bosses can be taken out with Level 2 Black Magic pretty handily, Warriors can equip almost anything, and White Mages never go out of style, so why bother with the other 20 or something jobs?
I do think that FF16 shoots itself in the foot a bit with its equipment selection which is basically a whole lot of junk. There are the accessories that make different skills do increased damage but they're so pointless and so plentiful that they just clog your inventory, while there is literally no difference between weapons other than stats (why are you throwing three new weapons at me, craftsman? Two of them are literally pointless) and the one time I got a sword with different attack and stagger damage, there was no equal and opposite sword to make the choice interesting.
I still enjoy the combat, but I'm not as skilled a player as you seem to be (I struggle a bit with the dodge mechanics so I keep the Focus Ring on for safety and I just gave up entirely on Torgal mechanics for this playthrough). It would be fair to say that the game is easy and definitely balanced toward players who aren't good at action games (like me!) and I think it would be fair to have wanted the option of a more difficult experience out the gate instead of restricting it to new game plus.
That said, I'm not done with the story (only about 75% of the way through but most of the table's been cleared sans a couple of villains), so I can speak to that a little. The overarching story is okay, to a point, I will agree with Greg at most points that it's undercooked. By far the most intriguing plot point (Dion Lesage is the MVP of the entire game thus far nobody come for him) comes to a head around the 64% mark and immediately ends with the biggest one-two punch of "reveals" that suck the majority of the air (or heh, aether) out of the story.
I wouldn't say it's "not-very-good" but rather that it's "flawed but engaging". By standards of the series, I'd still rank it closer to the top despite its flaws, but not nearly as strong as FF6 or FF9, certainly not FF Tactics. But where it shines for me is with Clive, who is still in the running as my favorite protagonist in the series, and I personally feel really engaged by the supporting cast. I don't mean the other dominants the game marketed as major characters (which sans Clive, Cid, Dion, and maaaaybe Jill has been an utter letdown) but the supporting characters that run the hub. It can come across as filler at times when you want to get to the next Eikon fight, and there's an egregious stretch around the 70 percent mark where you URGENTLY HAVE SOMEWHERE TO BE but the game throws a bunch of sidequests at you WHEN YOU HAVE SOMEWHERE YOU REALLY NEED TO BE RIGHT NOW. There's a good lull in the story right after this, why couldn't they pop then?
I'll weigh in on this aspect more when I finish the game and have some time to separate my thoughts, but there's a found family vibe that I resonate with to the hub and some of the surrounding villages. Like, there's a LOT that needs unpacking there considering the really boneheaded approach to diversity all things considered, but I resonate with the story a bit more strongly from an LGBTQIA+ perspective (to be clear--that's 100% my "interpretation" based on personal takeaways, does not seem to be writer-developer intended at all) as well as a sort of generational/family trauma story.
That latter read is precisely why 64% makes me so mad, but I'll dive deeper when I have the opportunity to digest the whole game and unpack the fallout from wasting the game's BEST antagonist.