Corollas have a very economical maintenace profile. i send my corollas at toyota western/ KArachi on every 5000 km for inspection and service.
only thing they change till now according to mileage is as follows:
Oil filter + Oil every 5000km
Air filter checked at every 5000 but replaced at 20,000km
brakes : serviced every 10,000 and front pads replaced at 50,000. (i checked the old ones , they could have done a 15000 km more)
Plugs : changed at 50000km. ( factory fitted ones )
Rest is all fine. it doesnt even need tuning every 5000 km. they refuse it themselves saying , its all good, dont mess with a perfectly tuned car.
i am satisfied by their service and cars performance.
Same cars when sold in US or JApan market undergoes even lesser maintanece checks, as maintenance is expensive and it gives a bad reputaion to the car for requiring parts every 5 to 10 k km. only exception is for oil at 5-7000 miles.
I agree roads are bad in Pakistan and more dusty too, but city conditions arent that bad.
In my experience and after talking to several mechanics, all advised to keep the original plugs. other performance plugs ma cause more engine head pressure and heat produced which can ultimately lessen the engine life.
i have maintained cars with their original engines, gaskets, radiators till 300,000 km in 3 cars. All without performance parts.