Hi all, wanted to post final resolution here. Turned out to be a problem with a relay that was supplying power to the ECU. Attaching pics of the culprit. Once this was changed, car is as good as new.
Few learnings / tips :
1) One cannot trust even big stealerships such as Toyota. They wanted to completely strip my engines wiring which I refused. Please be cautious and do not blindly follow proposed solutions by electricians and mechanics. Ask around and do research online for plausible solutions.
2) Another big workshop here in Karachi immediately wanted to replace my ECU without even looking at the car. That was a big red flag for me and I followed my gut feeling of not even having them inspect the car.
3) Imported genuine repair manual, made a few phone calls to reliable people I knew who referred a competent electrician. He referred to some wiring schematics from my repair manual and was able to identify and fix the issue within 30 minutes.
Conclusion: Mostly quacks out there calling themselves mechanics / electricians - in order to protect your car, investment and sanity, please always do your own due diligence.
Thanks all to those who helped!
