If HEX is not running due to engineering work, then the Elizabeth line is also affected, as it uses the same tracks. The TfL engineering work calendar shows no Elizabeth line between Paddington and Ealing Broadway before 0745. Thus the first EL is shown on the journey planners as at 0743 from Paddington.
Before then use the District or Central lines from central London to Ealing Broadway.
Engineering work can and sometimes does over-run- that would lead to later first HEX and Elizabeth line trains.
Maybe consider using National Express coaches from Victoria if really worried
9.1% of trains cancelled is 1 in 10, meaning that 9 of 10 ran. Sensationalist journalism. What the report stated, though rather glossed over (as usual) by the BBC, was that the vast majority of the cancellations were due to infrastructure problems, not train faults.
Those infrastructure issues would also have affected HEX and GWR main line trains if they happened between the airport and Paddington. They couldn't not have done so.
There is another thread which says something similar about the Piccadilly line- that it is the least reliable tube line!!