But... you haven't moved it up. It's still in A.
I'd also like to point out that Scarf Nihilego reliably deals with it. It can switch into any unboosted attack, even HP Ground (increasingly uncommon these days in favor of another coverage move like HP Ice, which you mentioned), and then reliably outspeed and OHKO it with power gem. Even if you have charti berry, Nihilego isn't reliably OHKOed by any of Volcarona's moves at +1 (besides shattered psyche and you can't have that with charti berry so), so it can take get rid of the charti berry, then take a hit, then finish Volc off. I repeat - the only move of Volcarona's that reliably takes Nihilego out in one hit (to my knowledge, there's a possibility I'm wrong) is +1 shattered psyche, so Nihilego beats even charti sets. It's more niche than Chomp or Keldeo, but it's still in B+ and thus still pretty good.
I'm not saying that it's anywhere close to a bad mon - Volcarona is terrifying and every team besides like HO should have an answer to it. But there are switch ins to it, there are things that reliably handle it (besides hax or charti berry, and charti berry means it's not running a z move). But it needs support - lots of it. It needs a hazard remover, it needs a teammate to get rid of the scarfer on the other team, etc. Imo, you're downplaying how much that affects it. It's similar to CharY in a sense that there's CharY teams, not teams that include CharY. CharY is more extreme because it takes up the mega slot and usually is accompanied by dug (as well as fini or another remover), but I think they're still comparable. While you *could* build a team around any of the A+ mons, they're all splashable enough that they can fit on teams not built around them.
Nevermind the fact that it really only has the one QD set, differing only in items and coverage. Every mon in A+ bar Ash-Gren (to a degree) has multiple different and threatening sets. Mawile has AOA, SD, SubFocus, etc. Celesteela has SpD and Autotomize. Zygarde has DD, SubCoil, Band. Unpredictability is a real thing. Volcarona has QD+STAB Fire+2 coverage moves, along with a few z moves, leftovers, and charti.
Ash Gren has Specs, really, though I guess some people run Taunt and/or Life Orb. But Ash Gren is stupidly good at what it does (like Volcarona) and is splashable as fuck while requiring little support to do its job (unlike Volcarona). It still has a bit of unpredictability too due to protean Gren.
I really think that's the difference between an A mon and an A+ mon. Ash Gren and Volcarona are both extremely good at what they do, but one requires a large amount of support, while the other doesn't.
You quiver again on the power gem which you live due to charti and a +2 psychic does like 90-106 I believe and if you run HP ground that kills. Regardless, I do understand your counter argument but the list of fail safe counters to this thing just seems too small in my opinion. Also to note, the charti sets I have seen haven't run psychic or hp ground but I guess either one would still be a viable option