Alex Pereira's double champ achievement is impressive, joining a select group, but he lost the Middleweight Belt in their rematch, I think seeing him getting back to the Middleweight to get the belt back would be awesome for sure he is not tested yet with wrestlers of Khabib Nurmagomedov level and for sure Magomed Ankalaev will be a great test for him and just like you I am also seeing a new champ in the Light Heavyweight for sure, but Jamal Hill will not going to last long in the stand-up if he would face Alex Pereira.
Pereira has actually fought fighters with decent ground game, be it Strickland, with a solid wrestling background, or Blachowicz, a jiu-jitsu black belt. It's just they didn't capitalise on their ground game advantage. Strickland got knocked out and Blachowicz only managed to get 3 takedowns which led to nothing.
So it's hard to say if Pereira has been tested or not. There are no wrestlers of Khabib's calibre in the light heavyweight. Ankalaev is also a wrestler, but he's not anywhere near Khabib's level. Still, he's good enough to take Pereira to the ground and drown him there, that's why I expect the UFC will prefer to move Pereira down to the middleweight than to get him to fight Ankalaev.