In September it will be like +28|+32 С degree at day time and about +25|+27 С degree at night. If you are OK with rains what usually are very intensive but short (30-60 min), then you'll be OK. Sky will be mostly cloudy.
For me September in Pattaya is for sure better than April-May.
Hmmm
I do think September would be better then.
I do prefer actuall rain instead of humidity in the air.
And +/- 30°C is okay to me.