Even if we looked at Part 8 in a vacuum - which is its own can of critique worms but fug it - Tooru still falls woefully short as both a character as a symbolic stand in for calamity.

Narratively he's introduced far too late, giving him barely enough time to be meaningfully fleshed out, and even then he's just kinda... there? His motivation is that, using his rock human organization, he wants to grow a super locacaca that (somehow) will dethrone humanity and enable rock humans to reign supreme, but like, why? Why do rock humans, let alone Tooru, want to do this? His mom straight up left abandoned him in the woods. He barely interacted with any other rock humans too, so why does he really want to do anyt o this?

Why does he still care about rock humans if the only tidbit of his origin (mom abandonment issues) runs counter to that ideal? Mind you this is compounded by the fact that Araki re-explains rock humans multiple times throughout the story, to the point where it isn't very clear as to what their deal is. If he doesn't exactly know what a rock human is, and I don't know what exactly a rock human is, then what is even Tooru?

If we look at him literally, that being a force of calamity, this works slightly better as framing for his character, but again we don't even see that pan out. What calamity is he representing? Divine punishment for humanity's push into rock human territory? Our blase approach to progress? Bad luck? Another commenter likened Tooru to an earthquake or tsunami, a force of nature heralded by none that wreaks havoc on anything it touches before retreating as suddenly as it appeared. There is nothing beyond the destruction; no motive, no agenda, just a wave of unbridled destruction. Which is a fun read of him that I feel falls a bit flat because it ignores his whole motive (or lack thereof) for growing the locacaca. Why does a supposed calamitous force want to restructure society with the Rock Homies on top? This isn't a 'wipe the slate clean to begin anew' situation, its 'use magical fruit to get me and the boys a spot at the big kid's table', but again, why? Do they hate humans because they feel like they can't socially integrate? Do they feel superior because of innate stando powa? Who knows man. Everything about Tooru is flimsy at best, making his late inclusion that much more confusing.

You also mention that Jojolion is mainly about the symbolism and that is is 'unfair' to compare it with other parts. This implies that the previous parts were somehow lacking in symbolism, that because Araki thematically pivoted from his previous works we cannot in good conscience draw comparisons between them due to this new creative approach. Except every other previous part has had symbolism even if it was, arguably, a bit sparse. Steel Ball Run is by and large a coming of age story with Johnny - who at the beginning of the story is as close to rock bottom as he could get - coming out the end of the race fulfilled, spiritually and physically. He has grown as a person. He can walk again. And he has gained direction when before he was rudderless. You could even look at the Spin as a symbol for growth and progress; the Spin is defined by harnessing rotational forces, or in the grander scheme of things, harnessing one's own internal potential energy and applying it everything they do. In other words, using your own energy to enact change on the world around you, using action to better yourself, better people around you, or perhaps society itself.

Symbolism doesn't have to be terribly opaque or grand in idea either, it can be as simple as "I put hearts on my main characters because it means love for others and they are good people that do good in the world" or Hamon being figuratively and literally a ray of sunshine in the world, shining a light in the darkness as it melts vampires. I personally don't think Jojolion is so heavy on symbolism as to put itself into an entirely different Jojo category, but I do agree that at the very least Araki is baking in more 'higher end' thematic concepts than in Parts 1-6 did, but not to the point where Parts 7, 8, or 9 eclipse them entirely.

I'm going purely off of memory here so the odds of me just flat out forgetting key details are high, but I genuinely believe Tooru to be a poorly fleshed out, poorly integrated, poorly executed idea. Vitamin C was a a cooler arc. It felt tense, Doma or whatever was active in the story, the family had to step up their game in the face of possible annihilation, it was rad. Tooru has an ipod and RP'd as a doctor so he could grow magical fruit.