You need a neck reamer then and not a neck turner. Why take brass off the outside of the neck when it is forming at the shoulder/neck junction? The doughnut will still be there.
Usually you only need to neck turn for a tight neck in the chamber or possibly to lightly true up the necks on some cases, but if it is all down to flow, it will just re-occur anyway as the brass flows forward into the neck. The errors in concentricity start back in the case body where one side might be thicker than the other and this simply extrudes forwards, so it will re-occur evetually if you just neck turn.
If you are worried about incipient head seperation use a piece of wire with a bend and run it over the inside of the case, you will feel it before you see it by which time its replacement time.
I have had 2 cases of head sep.
In .303 where I FL re-sized. Neck only prevented it and I assume the chamber dims were a bit generous as the head spacing was ok
In 5.56x45 where I swapped FL dies and did not set the new one up properly and over bumped the shoulders back. A Dillon gaugue soon showed the error and it was sorted.