by the way, which is a better method/way to retard the timing? adding 0.1mm head shim or using a colder plug? and why?
Yes, I meant 0.1mm shim.
For the issue that I experienced, changing the plug to a colder one (in my case a 7 to 8) didn't help at all, in fact made the situation worse. Increasing the head clearance with the shim did the trick. Now my plugs seem to last forever.