Jevyod, so far so good. Your testing where expander is removed from die would suggest that you do not have a headspace issue. The FL die as installed in press where shell holder bumps die base, camming over a bit, sets back the case shoulder and sizes the case body taper to where case will chamber ok.
I assume you don't have tooling to measure the shoulder setback after FL sizing so an assumption is that the die is manufactured to give an acceptable shoulder setback where you are not excessively shortening the case.
I use bore mops to clean inside case neck, both before sizing and after sizing. No brushes here as I do not want to rough up the surface of the case neck. My preferred solvent is PVC pipe cleaner. Kinda nasty stuff but does remove loose powder fouling, case lube, and evaps proper. I suppose Isoalcohol or other solvents are good to. Pardon my rambling here.
I suggest you examine expander for any roughness. Polish expander might be a good option too.
