"... the bullet should be as concentic as possible to the outside case diameter."
Understand fully. And agree in concept. Share my experience just for food for thought.
What I did for a holder was to counter bore a (3/8") brass 3C collet about .475" wide and .25" deep, leaving a sharp shoulder to index the case head against. That part worked good.
I then chucked a tapered pilot into my tail stock's high grade 3 jaw drill chuck. Used that pilot as a guide to align the case before securing it firmly in the collet. Removed the pilot and inserted a conventional (Forster) neck reamer. Slowly fed the reamer into the centered necks (which had already been sized and expanded to leave the interior slightly tighter, maybe a thou, than the OD of the reamer). The interior cut worked good and, as you note, that made sure my neck was both smoooth and the center line was in direct alighment with the CL of the case.
After boring the neck, and without moving the case, I swapped the reamer for a snugly fitted (and well lubed) pilot as a work mandrel and then just turned the necks normally.
It worked pretty well but, for me anyway, annealing and using a conventonal outside turner, followed by a couple of hot firings, has done just as well.
It was fun to try but with no real gain for all the sweat and tears on the lathe, I did not repeat it! Maybe you can improve on it?? If so, I'd love to hear your method and results.