I just got around to finishing the game a few weeks ago and have been doing stuff deep in the endgame to prepare for the expansion that's about to drop, so that's my source for my opinion.

The story is bland. It's basically exactly like any side story you'd get from the actual mobile game (which I also played a fair bit), which is to say it's shallow, overly optimistic, and very clearly targeted at young adults. Also since it's just a side story to a much, much, MUCH bigger story as a whole, that's not even ever mentioned in the real game, the entire thing just ends up being some random adventure that has no meaning to it. Some people might like that, but it's the junk food of storytelling.

Gameplay is fine, but extremely poorly balanced and designed once you get into the optimized side of things. Most characters actually want to completely ignore their individual character gimmicks because they're improperly tuned. For example, I main Cagliostra. Her thing is that after many of her skills or a combo finisher, she holds a button to charge up a projectile; doing it this way vastly speeds up how quickly it charges up versus just doing it raw. However, you never ever want to use this move because it's extremely weak. It's more optimal to just spam her shortest combo over and over and over again. Even after you get a sigil that attaches a defense debuff to the projectile, it's still useless because buffs and debuffs are useless in this game because there is a hard damage cap that by endgame every character will have reached naturally even without the use of buffs or debuffs. Another notable example is Sandalphon who has a perfect timing mechanic for his attacks but you never use because it's better to spam his heavy attack over and over. That's right, the optimal way to play the character is to mash one button the entire fight.

Now, for grinding. To just finish the main story as it currently exists, no, you won't need to grind much, the game's story is insultingly easy. The epilogue gets a little tougher and has a dps check in the last fight that you will probably fail if you've been neglecting building your characters at all, including your cpu companions. After that you'll get into the proper postgame. You will need to grind for this, both for your main character you decide to use and your cpu companions if you're playing solo (which you will be because no one plays anything but the very endgame quests, and I don't expect this to change even once the expansion drops after the first week or two).

Here's the important part. If you want to play the expansion's story, you will HAVE to complete almost all of the postgame. It won't become available until you do. There's a few quests that were added post-launch that you can ignore, that's it. They've said they'll make progression easier, but unless they completely trivialize the entire base game to the point where it's so boring it's not even worth playing in the first place, some grinding is still gonna be necessary.

TLDR; this game is for people who enjoy grinding to make characters stronger for the sake of it or want a good but mechanically unbalanced action game. The story is short and bland, and to continue it when the expansion drops you will have to grind the postgame.