Not sure if you're still using this Dusclops set, but an EV spread of 252/64/192 with an Impish nature gives you about 8% more bulk on both sides, if my calculations are correct.
I actually haven't gotten around to testing it yet but thanks for this! Not the best with calculations so appreciate the help. Is this for level 50 or open level? I was working on the basis that if it survives Slaking3's Shadow Ball (as well as Crobat3 and Dusclops3 after Screech/Swagger respectively) it can survive pretty much any other physical move. Houndoom's Crunch is probably the most threatening special move out there, but there's not much Dusclops can do against Houndoom anyway short of using Destiny Bond for the double KO next turn and praying you don't flinch. Houndoom in general is a pain for this team if Dusclops is on it; two sets have Crunch, while sets 2, 3, and 4 KO Metagross for certain - it has a one-third chance of surviving set 1's Flamethrower, but that's not much consolation.
I've actually been thinking lately Metagross should run Jolly; the decrease in power is annoying, but you still OHKO a lot of extremely bulky stuff with Explosion (and actually still just manage it against stuff like Scizor). It would allow some investment in Special Defence because tbh there's not much around 122 Speed you'll be desperate to outspeed, but then again, max speed gets you to 134 which outruns Blaziken3 and several Tauros variants...
I agree with your reasoning that this is probably the best Ghost option, but I wonder if other Pokemon might be able to offer additional utility that compensates for not being able to status an incoming Pokemon after Explosion KOs one or both opponents.
Slowbro seems very solid, but I feel like you might miss Sableye's Fake Out and Dusclops' utility.
A weird suggestion I have is Thick Fat Dewgong. It can use Protect/Water move/Icy Wind/Fake Out, threatening (at least some of) the Fire mons that Metagross doesn't like and replicating some of the post-Explosion utility that the Ghosts have. Stats definitely leave something to be desired though, as do the weaknesses of the Ice type. Also, egg move incompatibility strikes yet again: Perish Song and Encore could be fun options, but neither is compatible with the other, nor with Fake Out.
Hmm, Dewgong's an interesting one. I feel like it's a Pokemon I often underestimate but it's a better mon than it looks on paper for sure. It's usually not one that's completely safe to face at the very least. I also had Kangaskhan in mind as a Fake Out user but it can't really achieve much outside of that.
Water typing is useful though because it covers several of the Rock/Steel types that don't fear Explosion - though as you alluded to it's not powerful enough to outright OHKO, especially when you factor in the spread damage penalty for Surf. Aggron is a 3HKO with Surf; you could run Waterfall for a more sure hit, but you'd still need to go Modest 252 for a slim chance of an OHKO and investing in defences is definitely more worthwhile. It's crying out for Hydro Pump really; still, there's always Sheer Cold, I guess...
My thinking is just that running anything that isn't immune to Explosion and therefore needs to spend time Protecting is such a waste, since ghosts have the freedom to take their turn in the face of Metagross blowing up (particularly if you're in a scenario where you do need to blow up on turn one). That said, Dewgong does get Dive which would of course enable it to avoid being hit by Explosion. But then if that was the play I'd be more inclined to go with Suicune instead and that's better suited to the backup slot.
Ugh, decisions. This became a bit of an essay, apologies.