Your post looks good in general but it lacks the one thing that actually matters: you're not limited to only 1 shell smasher.Got some (hopefully) hot takes here on some of the problematic elements so far. I feel strongly that Shell Smash as a whole is not broken, and we shouldn't be entertaining banning it as a whole. It would be a really weird decision, since there are very definitely non-broken Shell Smashers. I'll take a look at some individual Shell Smash users, but the move itself is not really a problem imo.
Cloyster: It boasts a lot of power after a Shell Smash, that's for sure, but the fact that it has such unilateral coverage means that most water-types can stomach a hit from it. Cloyster's biggest limiting factor is that it's slow as hell. At +2 Speed it is still slower than any Scarfer with base 85 Speed or higher, meaning Scarf Diggersby or anything faster can revenge kill it. Non-scarfed Snorlax outspeeds it too. Not to mention priority, something all Shell Smash sweepers struggle with to an extent. Admittedly, it can feel a bit overwhelming if you're running a more defensively oriented team lacking bulky water-types, but offensively it is very easy to check with any good scarfer or priority mon. The fact that it is so matchup dependant really prevents it from being consistent enough to be considered broken at this stage, to me.
Blastoise: Seems good, but faces a lot of the same problems as Cloyster does in that its coverage is fairly limited and reliant on its Water moves. Unlike Cloyster, it is fast enough to outspeed all scarfers bar Snorlax at +2, but it lacks a lot of power in comparison due to its lower offensive stats and single STAB. Keldeo, Virizion, Dracovish, Gyarados, Kingler, Dragapult, depending on set even Roserade or Araquanid are all mons that can take the hits and threaten back. It feels like a cop-out to mention priority, but when some of the most powerful mons in the metagame with little opportunity cost for running like Sylveon and Diggersby have very powerful priority, it goes a long way towards preventing these Shell Smashers from feeling too strong, at least to me. Blastoise is definitely more consistent against offense though, but in my experience it never felt like it was too much to handle.
Turtonator: Haven't seen it yet. On paper it seems generally inferior to Torkoal, being slower and weaker, but it can hit Dragapult and Hydreigon harder with its Dragon moves. It is slow enough that anything faster than scarf base 97 can outspeed it at +2. Not much else to say about it.
Barbaracle: I have not encountered this yet either. Seems mostly worse than Blastoise honestly, since its special movepool isn't that amazing.
Torkoal: I feel like Torkoal is the only Shell Smash user that feels potentially too good to me. Ridiculously strong with excellent coverage in Earth Power and Solar Beam, it's an amazing breaker or slow Scarfer even if you ignore Shell Smash. The fact that it has these alternate sets (mostly Specs which has no switch-ins) is part of what elevates it. Fast enough that you need a base 110 scarfer to outspeed it, Torkoal is one of the main reasons you would run scarf Rhyperior or Coalossal. It's super strong, able to wash bulky resists like Hydreigon at +2, and although it's frail, the fact that it resists Sylveon's Quick Attack makes that a bit more limited in dealing with it. Again, like the other Shell Smashers, priority or scarf Ditto handles it, but I always find myself having to prep for Torkoal more since it has fewer defensive AND offensive checks. I feel like Torkoal is really the only Shell Smasher that makes a case for broken cause of this versatility and the fact that it pushes speed and power to a higher level.
TLDR: Suspect Torkoal instead of Shell Smash imo
You can run Cloy + Steelix/Rhyp + Torkoal for a triple special threat that nothing in this meta can tank out, at all.
That's why Turtonator is more relevant than you make it out to be; without sun it's weaker than torkoal but in sun it does everything Torkoal can't (kill pult + drei without crit/2hko luck/SR necessity) and in tandem with koal and or cloy it breaks teams. This still applies if you just use Ninetales for Sun and then set up, it changes barely anything.
Shell smash is the broken factor in this.
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