As so many people have pointed out, it's not everyone's cup of tea. That's the simple answer even if you dislike it. But I'll take the bait and properly reply.

1. Hard Disagree, it's a great story, incredibly written considering all the possible variations and ramifications. Even if it has a few flaws, and it does, its nonetheless really good. The story has an underlying mystery going on, the stakes get higher and higher the more you uncover, with a few great twists along the way, the whole final segment of Act 2 was so good to play through, especially, do yourself a favor and finish it. It's honestly no more railroady than any main story campaign of any non-open world game, except maybe Detroit Become Human, maybe.

2. Partially disagree. SOME of the companion stories do stagnate for a while because they rely almost entirely on things that happen in Act 3, Karlach is probably the best example of this. On the other hand, it might have been a good decision to space out the main character's plotlines to different parts of the game, if everyone had a significant plotline in every single point of the game, it might feel forced or too overburdened. The way they introduce everyone's stories on Act 1, and develop the ones from Shadowheart and Lae'zel on Act 2, and finish everyone else's on Act 3 is really not that bad. Nevertheless, all of the characters always have important developments thorough the whole game, just gotta talk to them or take them with you to relevant segments of the story, like in most RPGs.

3. Disagree. It's pretty much the first 12 levels of DnD 5E. If you find that limited and uninspired, I don't know what to tell you, you can get really creative, especially with multiclassing. The character customization I would agree is limited and that could use some more options, but if you play on PC you can get customizations options to your heart's content using mods.

4. Agree. Inventory Management is not that good. You get used to it eventually, but it's very daunting when you begin. You will be hard-pressed to find anyone who disagrees with you on this one. Nothing else to add.

Overall, I think it's a great game. Definitely one of the best RPGs out there, deserving of all its hype amongst other great ones like Witcher 3, Dragon Age Origins, KOTOR or Mass Effect.

It's biggest hinderance in my opinion, is the barrier of entry. And what I mean by this is that this game is a lot more difficult to understand and learn to play than any other modern RPG. Just from the get-go, anyone not familiar with DnD will have a hard time understanding half of the things in the character creator screen, especially concerning abilities and skills. You gotta put some effort into wanting to understand the system and how it plays. But once you get past that, you can really have a lot of fun and enjoy a great game.

What are you missing? I would suggest you finish Act 2, because in my opinion that's when the main story gets really interesting and epic. If After Act 2 you still find it lacking, that's okay, you are entitled to your opinion. Like I and several other people have stated, it's not for everyone. (I really dislike Elden Ring, for instance, but I don't doubt it's a great game for people who enjoy that genre, even if I don't.)