I basically use standard FL dies (forster and redding) with the expander balls removed as they are horrible and I dislike the graunchy feeling when pulling them back through. It just strikes me as needing too much pressure which i cannot see being good for pushing and pulling the necks about. I don't use any bushings. Just FL dies to bump the shoulder back 2 thou without the expander ball and then use an expander die and relevant mandrel to size them back up to desired size and neck tension.
The process of forcing that mandrel in and less so, out, of the case neck is much less severe force wise which I can feel as I operate the press, than if I was using the expander balls supplied with the FL dies.
My process is - I decap all cases first with universal decapper, then I anneal, then FL size without expander ball and without cleaning the cases but I make sure they are free from obvious debris or dirt. I then clean them in the ultrasonic cleaner to clean the internals and the primer pockets and to remove resizing lube (I use that cheapo Lee stuff) and then I use the expander mandrel to size necks up before chamfer/deburr and then on to priming, charging and seating bullets.
The process for lubing the inside of case necks prior to using the expander mandrel die, is to dip maybe 3-4 times in the graphite covered shot and then on to the press. I run the mandrel through each twice to ensure some sort of uniformity but I am generally annealing every firing, so using the mandrel and seating bullets has a very very consistent feel to it. That might help you as you effectively have softer brass to work with and it also provides longer life for your cases (apologies if you do anneal, I didn't read back through the thread entirely)
I recently helped a mate load up a crapton of 7mmRM cartridges. His reloading process is somewhat primitive, so I annealed all the cases for him and then got him to feel how the seating and mandrels felt in effectively virgin brass. He was amazed how soft it felt. You also get much more consistent measurements COAL seating wise, as the material has the same amount of give or friction as the next case. Unannealed brass which has been fired a coupla three time or more will display a lack of consistency in this regard. You can feel it on the press.
I don't measure my neck internals to see how much work the mandrel is doing. I am loading for .222rem, .243win, 6.5CM and 7mmRM. I work on the basis that if something works, it aint stupid. I did ponder if you had a FL die that was oversizing the necks down but you say there is a 6 thou sizing difference between sized down necks and then opening them back up. That does not sound excessive to me at all. I am going to measure a sized down neck in a mo on a piece of brass and will report.
So in a nutshell, I don't know. Sounds to me like you are doing it right. If you are not annealing and are working with heavily work hardened brass, that might explain it?
Right, here are some pics to show you my tiny lube pot. I very rarely top this up with graphite. A tiny amount goes a long way. You can also see the discolouration of the mandrels. The tin or titanium one (don't ask, I had to sell a kidney to buy that one as the standard version was out of stock) seems to hold less graphite but it still works the same. That discolouration is graphite and it builds up and assists the next case that goes through the die. I still dip every single case. The other picture shows you my clean finger tip and then what it looks like if I very very lightly press it againt the graphite covered shot. I have never cleaned or done anything to those mandrels. They are just stored in a dry, clean place.
On to the measurements. Just measured the internal diameter of a 7mmRM case that has been sized down using a FL die without expander but is yet to be sized back up with a mandrel. It is 0.272" and the mandrel would be 0.283" so that is being worked by 10 thou or so which is more than your 6 thou. I don't think either of those are out of kilter per se.
Best of luck