Sir:
I have to agree that your problem with bullet tension in the case is due to the expander button being too large.
Your full length sizing die has a stem inside it, that holds the de-caping pin and expander button. When you size the case, the neck is sized much smaller than necessary, as the case is removed from the die, the expander button brings it to its proper size.
If you remove that stem from the die and mic the button it will indicate its size. Depending on if you are using lead or jacketed bullets, that button should be .002 or .003 smaller that the bullet diameter. You can purchase expanding buttons of various sizes, or if you have the tools that one can be reduced to the proper size. Jacketed bullet diameter should be what you slugged groove diameter indicates and if using lead bullets they should be around .002 over size.
I don't believe you need another die, just alter the expanding button.
Bob