Well I'd say the issue isn't so much his lack of a grand noble goal. He's a villain. Noble intentions aren't expected. As for the "grand" part I'd say he has that. He wants to bring down hero society.
It's his motivations that are problematic. Because at the core of his motivations is the idea that "he was failed" right? The problem there is that isn't a motivation to destroy society but rather fix it. Because the trauma that shaped him is that "heroes aren't good enough" as opposed to "heroes are bad", which the series acts like is his motivation. The only way to make that work would have been to make it a specific person that failed him who he's trying to kill. Something more personal. But it's not personal with him. It manifests as something like an ideology but, unlike with Stain, his ideology doesn't really make a lot of sense. Even by crazy character internal logic.
I think it would honestly be better if his motives were non-ideological, like money or personal power, because those at least make sense. And can serve a counterpoint to selfless heroes.
Really what Boku no Hero needs is a villain who can serve as a counterpoint to the heroic worldview. To let real selfless heroism shine. Like an anarchist who believes people should be able to use their quirks without being bound by rules. Or a megalomaniac who wants to use his power to achieve world domination. Or a villain that's pretending to be a hero for his own benefit. And Shiragaki just isn't that. He's like a kid throwing a tantrum. Now matter how powerful he becomes, to me, that's just not very interesting and I'm not really sure how you fix that.
Agree with all of this. This is why a lot of people in the fandom believe AFO makes a better villain for the story than Shiggy, and why many also believe he’ll succeed in hijacking Shiggy, if not now, then maybe at the endgame when Deku exhausts all of his power and energy taking him out.
I would also include Dabi as well, if (for the love of god, I hope) he really is Touya. Because Dabi’s reasons would connect to the readers more than Shiggy’s would. His hatred of heroes and the system would come from a much more personal experience, and it connects better to his villainous goals. By tearing down everything Endeavor’s worked so hard to protect, he tears down everything Endeavor is, while also setting up Shouto to be the one to stop him, not just for the sake of Japan, but also for his family. Which is, ironically, is exactly why Deku calls him MC material.
What Horikoshi seems to be trying to do is write him as the Joker, but the Joker never had someone string him along, but he fully embraced his madness not because of his past, but because he’s accepted his identity. Hopefully that makes sense.
Shiggy on the other hand, was completely molded by AFO since an early age, and like any child would do, he just obeyed and heeded AFO’s words. Even his backstory was potentially fabricated. With all the power and resources AFO had, you really wouldn’t think he had a hand in the Shimuras’ deaths? That’s why Shiggy makes a less compelling villain compared to other villains of yore.
Even though he is making strides in achieving independence from AFO, everything that he does still goes back to the man.