^^
This basically
Point 1:
Erza is a MC, she grows relative to side characters just by virtue of that alone. Erza has always had higher growth compared to Mira, ever since Mira recovered her lost powers against Freed.
The gap between Erza and Laxus has been reducing since day 1, and it is now the lowest it has ever been. Just because Erza grew so much doesn't mean Mira, a side character, grew so much
Point 2:
Erza has consistently outperformed and upstaged Mira, In every single arc. From Tenrou to Tartarus to Alverez to 100YQ.
She is as much of a rival to Erza, as Gajeel or Gray is to Natsu. Ok, I know that's too far, Natsu is THE main character and his growth rate blows everyone out of the water. But the essence of the point still stands.
Mira, Laxus, Jellal etc are all secondary characters, having similar roles in the series, their relative growth remains about the same, unlike Erza who has higher relative growth and Natsu who has insane relative growth.
Laxus/Jellal would defeat Mira with the same difficulty they would in BOS, a mid diff.