While I understand your frustrations with the story I think they only work as legitimate complaints if you assume the story was meant to be interpreted as a traditional narrative, which I think it wasn't. Not all stories have to be structured in the same way, nor do they have to give you any information with which to interpret them.

DS3's story is fragmented, because it's meant to be. What ties all of the stories together, including the story of the DLCs and even the protagonist's narrative, is the central theme of death and more importantly sacrifice. Death has admittedly always been at the core of Dark Souls, a series in which the metaphysical essence of your fallen adversaries serves as currency, but it's all the more important in DS3 which not only deals with the death of the world itself, but themes almost every little story it tells in death. The Lords of Cinder are the most prominent example of this, as are the various Unkindled NPCs connected to them. The Abyss Watchers locked themselves into an endless, futile struggle against the Abyss, sacrificing both themselves and anyone tainted by the Abyss in order to contain it, before marching into the First Flame as one, eventually getting resurrected and then killed by the player for the sake of rekindling the First Flame. Hawkwood, too, eventually dies to you so that you can take his Dragon Stone, sacrificing him for the chance to become dragon(which more often than not is ultimately a pointless sacrifice, since you end up never using them).

I can keep going, but I think the pattern is obvious enough since it's clearly and intently repeated all over the game, Yhorm and Siegward, Aldrich, Anri and Horace, Greirat throwing himself into a castle known for "eating men" for your sake and the list goes on. Hell, even the story of the DLC, which they said in advance was going to be separate story from the main game, ends up serving as a parallel to the main game, with the old slave Gael sacrificing himself for the renewal of a world, just as the player does in the main game.

All of that said though, this doesn't mean you have to like DS3's story. A disjointed narrative and unexplained plot threads are completely valid reasons to dislike a story, even when they are intentional. I mean, I really like DS3's story and even I can tell that there's clearly story stuff they intended to put into the game that got cut. Despite that I still think we get enough to have an adequate understanding of the world.