I've reloaded a lot of 6 mm Rem and always used a set of old C&H .244 Rem neck sizing dies that belonged to my Dad. I never had any problem with simply neck sizing, and have reloaded brass to the point where I scrapped it after seeing neck cracks after perhaps 8 reloadings. None of my 6 mm brass has ever been full length resized, and my Dad never full length sized any brass he used in his .244 Rem which is the same cartridge. The shoulders can only stretch as much as the chamber will permit, and immediately upon firing, they should retract or spring back a very small amount which will allow easy chambering next time. Is extraction normal? Have you tried rechambering the empties after firing, but before resizing? You might try smoking the neck and shoulder area to see where contact is occurring with your reloads.
I also never crimped my bullets when reloading the 6 mm. Neck sizing tension was always more than adequate. But I noticed that case length must be identical to get good crimps when I reload for my .45-70. Is it possible your crimping is creating a very slight buckling of the neck/shoulder juncture with some cases?
I do want to ask why you annealed the case necks after only two reloadings? I typically don't see any neck cracks developing until cases have been reloaded 7 or 8 times, and my groundhog loads are fairly warm.
