Well done!!
In essence, the starting point is to ensure that the draw in the flue is sufficient (-1mm H2O) if you have the ability to measure it - that will ensure that products if combustion are drawn away and that combustion air is pulled.
As long as the room the range is in is adequately ventilated you will always have sufficient combustion air to feed the burner.
The burner door should never be left open - it should also be adequately sealed. The range draws combustion air from the low level vents up to the burners. If the door is open, this flow is interrupted, but far worse is the danger of any blow back from the burner - the burner door is there to protect you and the kitchen in the unlikely event of that happening.
The above us also why it is difficult to assess the flame picture, because when the door is open the burner is not in its steady state. In an ideal world the burner door would have an inspection window.
So for improving the air flow, you modify the restrictor plate - to do this, make a second restrictor plate - a copy of the first in thin sheet steel and make slight modifications to see the impact. Once you are happy with it, copy the mods to the original restrictor plate.
You are trying to get a draw that gives you a dull red surface to the burner with a blue flame ( no yellow) immediately above it. Too much draw and the flame will start to lift off the burner.
In reality on an old stove, it will always be a compromise - but it looks like you have already achieved that - and now have a good knowledge of the idiosyncrasies of the range.
Personally, I would not use the engineer again who suggested leaving the burner door slightly open. Normally after conversion from wood (or solid fuel) to oil, you want less not more combustion air.
If it is any consolation, I am really struggling at the moment to get an Aga that I have converted from oil to biomass chips to perform reliably. A nightmare!!