For me, he needs to win all 5 monuments and all 3 GTs. GTs (excluding crashes) is a mere formality.
I don't think in order to be considered GOAT, he needs to win MSR or Vuelta if he manages to win other GTs/monuments several more times in a similarly convincing manner as he does now. Vuelta and MSR are both firmly in the bottom half of GTs/Monuments, ranked by prestige and can easily be compensated by any other race of the same category.
At the same time, I also don't think he needs to match Merckx numbers to be considered GOAT by the majority over Merckx just because times are so different now and it's that much harder to pull off the same results as it was in the 70s. If Pog wins somewhere in the area of 5 TDFs, 5 other GTs, 15 monuments and 2 WCs it's going to be enough IMO. If we ask who is the greatest F1 driver of all times, very few will say Schumacher or Hamilton - most will go with Senna even though his results are no match for the former two gentlemen.
Pog's greatness will be painfully obvious once he stops and we're back to business as usual - classics won by classics specialists and GTs won by stage-race specialists. And at that point, a fair few more people will be on the GOAT bandwagon than they are today.