If that's manifold vacuum at idle, its not really related to PV selection.
The PV number is like the step up spring in the Carter.
If the the engine has good part throttle fuel distribution and efficiency, a low opening like 6.5 might be OK. Looking at some of the Holley 4bbls used by Chrysler in the late 60s, most used 8.5 PVs. The Holley 3310 used by GM on some of their hottest factory engines used a 10.5 PV on the primary side.
Do you have a way to measure vacuum or MAP when driving the car?
As the throttle is opened slowly from a steady cruise, you'll see the vacuum drop.
AFR should go leaner and the engine should pull harder (accelerate more if on a flat road).
At some throttle opening and manifold vacuum, the fuel mixture needs to go richer for the engine to pull harder.
If you feel a flat spot, or a flat and then it goes as you open it further, definately try a higher PV opening point.
The fact the AFR stayed the similar under all conditions makes me suspect the PV may not be sealed against the block, or something like that.
Oh you described it perfectly clearly.
What is not clear is the terminology used in the carburetor world.
The idle system shouldn't be called that. The 'idle system' supplies the fuel under all low throttle conditions.
The main system only comes into action when there is enough air velocity in the venturi. At low throttle, whether its idle, or cruising at 40 mph, doesn't pull much air. Not enough to create a strong pressure drop at the booster wall. But since there is a strong vacuum under the throttle, that vacuum acts on the transition slot - which pulls fuel from the idle downwell.