Ah, I know that section of road.Yes it is fast road and I find it very hazardous, at least for the section where I'm going uphill. I don't cruse on it for a long time - it's a short climb to the top and downhill is not a problem). Also it has not caused any conflict so far as the other two lanes are often empty, and the drivers have plenty of space to overtake me. No one is honking me or shouting at me. When I take primary they seem to react in a more sensible manner.
Going downhill is not a problem, except with the very rare fast overtake and sudden stop on the red light.
What makes me uncomfortable is the over speed an inch or two away overtaking when there is two more empty lanes.
I've tried different scenarios in a period over 3 weeks, and I will keep to the primary. It is actually much less dangerous then the other two options and I do consider it as negotiating a hazard. I've tried the following and came to the conclusion that 99% of the cars on this section of the road overtake me in the same manner.
1. Secondary position: cars don't bother leaving space when overtaking even if the other two lanes are empty (which is the majority of the time I've been on this road);
2. Between both positions: the result is the same. They speed up so I have no time to move away. In fact this makes it more dangerous as I am taking more space and if there are tree cars passing (one in each lane, which happens rarely but it does) the car doesn't even have the option to leave me space.
3. Primary position: prevents the cars from speeding and eventually they overtake me with enough space (fully moving in the second lane).
To be noted: I don't ride in this manner on any other road and I was very hesitant to take primary on this road.
This is where I move from the left lane (on Old Town) to the one that joins the flyover (from Rectory Grove) : https://email@example.com,-0.1069335,138m/data=!3m1!1e3
It sounds as if you have already worked out the best solution, trying different approaches and the like. I would say that the fact that you are aware of the threat of close passes here and are doing active things to increase your safety makes you a much more aware and competent rider than many.
The last time I rode this I was traveling south to Croydon and the approach road from reeves corner was a little long and worrying.