I know I'm in the minority but I will give my daily insight on why it should be a Garganacl suspect test over Pao. They're both broken, they both deserve bans but is anyone really stopping the Tera Water Curse Nacl set? if you put Cloak on it [beats other nacls], it beats Gholdengo (Covert Cloak) which is on every team pretty much due to the increase usage of Nacl in general. Nacl's dominace has absolutely changed the metagame in the last 3 weeks, Covert Cloak / Substitute user seems like a must on any team if you're trying to ladder with any success.
You can chip Pao with entry hazards if it decides to go CB, you have priority moves, there's a decent amount of walls to play around with (Shoutout to Pex, Dozo, Fairy Nacl, and Forry). I'm not saying it's not broken because it is but it's more manageable then something spamming Salt Cure while having the ability to setup in your face where you can't status it.
inb4 'the hurrdurr but covertcloak bruh' as if Knock Off doesn't exist.
While I agree with everything you mentioned about Garganacl, I think you're undermining Chien Pao's longevity. Great Tusk is currently S rank and sees use on nearly every competitively viable team, so removal is not as far fetched for Chien Pao teams. SD HDB is also becoming more prevalent, so counterplay is even more limited depending on your team. Yes, priority can always beat it, but thats merely limited to Mach Punch from Breloom, Tera Water Banded Aqua Jet from Azumarill if Chien Pao is chipped, and +2 Sucker Punch from a Kingambit who's lost a few of its teammates. Additionally, out of those walls, you need multiple of them to even reasonably check Chien Pao, since outside of Garganacl, all of them are 2HKOed by Tera Dark Chien Pao, Forry is such ass right now, and the popular Tera Water Curse Garganacl still can't take on Chien Pao without accumulating prior boosts (and ofc chien pao isn't switching into Garganacl). So at best, you bring Garganacl, who is already broken and can still lose to Chien Pao thanks to BS Icicle Crash (or Tera Dark Crunch if Tera Water), or you trade the longevity of multiple defensive Pokemon on your team to simply check a single mon (who has just opened up a path for sweepers like Dragonite and Volcarona to win the game).
They both have some limited forms of counterplay, which in Garganacl's case is Trick and bringing in a strong wallbreaker before it can accumulate ID/Curse, since you can usually determine its Tera type based on the team structure, but its not as though Chien Pao has much counterplay. They are truly about equal.
Additionally, I think its worth considering the types of mons they are. Garganacl provides a ton of value in the builder defensively and really patches up team structures against the common CB Chien Pao abusing teams that are common right now, even if it as a mon is super broken and constraining on the builder. Chien Pao does not provide any defensive utility and just acts as a nuke button like Chi-Yu, but without bringing any of Chi-Yu's actual defensive utility. Garganacl could be more broken as an individual mon, but it contributes more to the tier than Chien Pao does, thanks to this defensive utility.
To be brutally honest though, I think this is a case of broken checks broken. Garganacl is one of the few things that can check Chien Pao when positioned right, and Chien Pao is one of the few things that can break Garganacl when positioned right. Both of them are stupidly restrictive in the builder, and I 100% would be okay with both leaving or either one being suspected, but I think that if one goes, the metagame will not be equipped to handle the other.
On a different note, how yall feeling about those Loom/Azu teams that are so common on the ladder? I really like how Azu and Loom feel right now, both are incredible in this metagame and match up well into common defensive cores such as Dirge/Ting-Lu and Pex/Garg/Dengo.