Lets take a look at the original report from the Welsh Government when this scheme was devised:
https://gov.wales/sites/default/files/consultations/2018-04/180425-no2-weltag-stage-1-a483-wrexham-en.pdf
On page 15 we see table 1. This shows there prediction of what would happen if they did nothing (ie no speed limits introduced). It shows that levels were predicted to fall to 31 by 2022 if no measures introduced (levels would fall as older cars gets replaced with newer cleaner engines or electric cars).
So levels of 27.5 during a pandemic when there is massive drop in the number of cars using the A483 is hardly a roaring success compared to the baseline of what was predicted to happen if no measures were taken. There are also probably more electric cars on the road than they would have predicted in 2017