So, while Spectrier has certainly become more potent, I don't see it as outrageously broken(yet).
The funny thing that I noticed with Spectrier is that it can LARP the stall breaker arceus set where D-kiss functions as a makeshift(albeit taunt-proof) Recover, Calm Mind, Shadow Ball (over Judgement), and Taunt.
But coming back to some checks, Id say Palafin and scarf Rapid Shifu come to mind first
252+ Atk Choice Band Palafin-Hero Jet Punch vs. 0 HP / 0 Def Spectrier: 285-336 (83.5 - 98.5%) -- 68.8% chance to OHKO after 1 layer of Spikes (Meaning LO recoil kills it after)
Alternatively, Scarf Urshifu Surging Strikes also works well
252+ Atk Urshifu-Rapid-Strike Surging Strikes (3 hits) vs. 0 HP / 0 Def Spectrier on a critical hit: 309-369 (90.6 - 108.2%) -- guaranteed OHKO after 1 layer of Spikes
You also have the pokemon that naturally outspeed it in the form of Deoxys and Iron Bundle(Granted a CM set probably can handle these if Spectrier has enough health and SpD boosts).
252 SpA Life Orb Deoxys Psycho Boost vs. 0 HP / 4 SpD Spectrier: 398-468 (116.7 - 137.2%) -- guaranteed OHKO
252 SpA Choice Specs Iron Bundle Hydro Pump vs. 0 HP / 4 SpD Spectrier: 312-367 (91.4 - 107.6%) -- guaranteed OHKO after 1 layer of Spikes
While Palafin/Shifu are checked by Giratina and Bundle and Deo having fundamental issues(in the form of being choice locked and frail respectively), the question becomes would you want to use 2 ghost types in your team? especially when you consider that Dark types have been gaining more prominence owing to the threat that is Mewtwo. Now you could use Toxapex in your team to check these pokemon (but now you are using a Toxapex on your non-stall team LOL). Conversely also consider that you could have teams which feature Palafin/Shifu with a Bundle/Deo, where Palafin and Shifu can safely pivot to these faster mons to threaten your team.
However, since Spectrier only needs 3 moves mandatorily in the form of Nasty Plot/Calm Mind, Shadow Ball, and Draining Kiss, it can easily slot in Substitute as a 4th move to tackle these threats. However, do keep in mind that these sets prefer leftover or boots more than life orb, especially because of the meta moving more in the direction of hazard stacking.
However, when considering the substitute set, it still is threatened by a DD Kyurem-White Set, while Tera Fairy D-kiss does not kill without a boost, meaning Kyurem can always setup one DD in front of a LO Spectrier which is behind a substitute
+1 252 Atk Kyurem-White Icicle Spear (4 hits) vs. 0 HP / 0 Def Spectrier: 352-420 (103.2 - 123.1%) -- guaranteed OHKO
252 SpA Life Orb Tera Fairy Spectrier Draining Kiss (60 BP) vs. 0 HP / 0- SpD Kyurem-White: 307-367 (78.5 - 93.8%) -- guaranteed 2HKO
You also have other priority such as Hamurott's sucker punch which can threaten it if it doesn't have Tera
252 Atk Samurott-Hisui Sucker Punch vs. 0 HP / 0 Def Spectrier: 306-360 (89.7 - 105.5%) -- 37.5% chance to OHKO
From a defensive perspective, I do see Garganacl as an interesting meta option to handle Spectrier. Tera Poison could be seen as a 3rd viable tera type on Garg(over Fairy or Water) for a set that completely shuts down Spectrier (However, the question now becomes if poison is really a better tera type for it over Fairy?)
Nonetheless, while certainly more viable and threatening than before, it is yet to be seen if Spectrier could be considered a banworthy threat.
I want to respond to this because while i disagree with the opinion that Spectrier isn't broken, I think a lot of your reasoning doesn't hold up when looking outside of the vacuum of a 1v1 matchup.
Palafin 
Yeah, Palafin is an ok check against it as a revenger. Urshifu as well but I have separate points for that one. When it comes to fast, strong Pokémon I dont think that available means of revenging them (no matter how consistent it might be) alone can be a strong argument. While Palafin has seen some success as of late, Its not been the
most common mon especially with how much it struggles into Giratina, Palkia Origin, and its hazard weakness when facing more bulky teams (which lean hazard stack as of now). Another issue with relying on revenging as your way of handling a threat like Spectrier is that most often it will be simply functioning as a trade, something H/O builds that Spectrier is on don't mind, and when facing a Jet Punch can even abuse with Roaring Moon and even more niche tools like Dragonite. This isn't to say Palafin doesn't have a good matchup into some H/O teams cause it 100% does, but looking at it in a vacuum as a neutralizing factor for Spectrier ignores a lot about building around Palafin (and how unpopular it is outside of certain players) due to how it creates a reliance on its priority for effective speed control, being the main way to handle Spectrier for the teams I've seen it used on recently.
Urshifu Rapid-Strike 
Again, in a vacuum Urshifu is a fine way of managing a Spectrier, but also suffers from the similar issue as Palafin in that it struggles a lot into other top tier meta threats. I do quite like this mon as a scarfer but it is also exceedingly not popular right now with Ocean being the only one I've recently seen load it in tour play, I think due to the fact that the style of team it fits on tends to struggle into the more dominant H/O styles right now, even if Shifu itself can manage. It has the same issues with Palafin (or any choiced attacker for that matter) in that the moves it clicks may inadvertently allow for the rest of the team to get the free turn they want, which is a very dangerous game to play. And overall for both it and Palafin, these are B and B- tier mons respectively, who have niches but are not really competing with the biggest threats in the tier right now, and unless something changes to make them more viable they don't provide the best argument for Spectrier not being broken.
I'd also like to point out you undersold an arguably better Spectrier answer then either of these in Deoxys, which is much easier to fit on teams and has many more good matchups compared to Pfin and Shifu, especially when its much more splashable on non H/O.
Kyurem White 
I don't really know why you mentioned this, as
1. Kyurem white is not the most popular H/O mon right now
2. it relies on the assumption that the Spectrier and Kyurem will be both +0 when first meeting
Both these together don't really make a convincing arguement as to how Spectrier under control. Not only would a Spectrier not be likely to be a teams answer after a +1 Kyurem white gets a kill as it would probably pretty clearly die, but if the Kyurem-White is the Spectrier answer it will likely have to take on a Spectrier that is already at least at +1, and Kyurem Itself may be weakened to hazards. Your vacuum scenario where Kyurem and Spectrier are in a 1v1 does result in Kyurem winning, but I think looking at more specific interactions like this should be done with more nuance.
Samurott is a fine shout but, most non lead Samus are Scarf anyways so a great revenging option regardless. Sucker also assumes Spectrier hasn't terad
Garganacl 
Garganacle is an interesting one, since its pretty known that the spdef set can't really handle Spectier. I think tera poison could be interesting, but it leaves you unable to use Garganacl's Tera as an effective panic button in response to Roaring Moon, Gouging Fire, Arc forms, Lando-I, etc and instead leaving you still vulnerable to 3 of the mons I just mentioned, all of which are part of some H/O teams running around that you would want your Garganacl to also be able to manage if needed.
In closing I think I see where you are coming from with this as Spectrier is not without checks, however I think you should be looking at these checks still with better reference to the overall meta and common interactions, because that is a huge limiting aspect of every Spectrier check you discussed. I believe that Spectrier
leans broken due to the fact that at the very least it is able to consistently trade, and is able to easily threaten clean sweeps if given 1 free turn.