I've been watching The Sensorites(1964), a very slow paced story but imaginative and very representative of how varied and ambitious the format of the Hartnell era was.
What impressed me here though was the quality and professionalism of William Hartnell's performance, it's quite different to how he drifts into playing it in subsequent stories, here he is very much more focused as an actor and is delivering a serious and straight performance free of the eccentricities and playing-to-the-children that comes later. Particularly striking though is the impressive demonstration from him that despite the perceived perception Hartnell has no difficulties, whatsoever, in his dialogue. Even in episodes 1 and 2 as he is called on to reel off some pretty shocking tongue-twisters (Molybdenum) and science to explain to the audience he does so flawlessly and naturally! This is a striking performance he delivers in The Sensorites.
So when did the strain of the scheduling, and his own building fatigue, begin to show? In later months Hartnell would eventually descend into gibbering cackling, becoming a literal dotty old man. Was it intentional, or more a defensive screen to cover for his building fatigue and hidden health problems?
I think any eccentricity and cackling from Doc One was indeed a character disguise for his problems.
As to the turning point, I think the Hartnell/Doctor Who documentary seemed to highlight Galaxy 4 as a story where Hartnell was messing up his lines badly.