Post up your compression test numbers.
Only then can you tell what the state of the engine is.
If you have not done this, then I am not sure what answers you seek from a forum.
sorry, i re-read your post. This is a very interesting problem.
I have never seen a glow plug fail like that, and so uniformly across three of different brands.
It sounds like with that many KM’s on it, you are doing well.
That is the performance i would expect in a coaster bus. The only thing i would change is to go to 20w-50 oil.
I think what you are running is a bit thin for the age of the 1hz.