Dave,
I thought this was supposed to be fun
We all have had our problems, that never stops and is how we learn.
Shimming should not be necessary, although it can be done to purposely off set a cut.
Lathe chucks have a master jaw, usually a mark of some sort next to the screw identifying this, I always snug this one first, then snug the others around the chuck with a final tightening of the master.
Does this make a difference?, I do not know, but it was how I was taught and it does keep our methods consistent which is usually a good thing.
I also always wipe the clamping surface of each jaw with my thumb to be sure nothing is on the face of the jaw to offset the material. As you can imagine it would take very little to cock the piece substantially 6" out from the chuck.
Make sure no burrs are on the shaft end as well.
It is also good practice to not just slap the material in the chuck and tighten away, but hand position it as you snug up the master jaw.
6" sticking out is far enough to introduce some flex and to be supported with a center would be a good idea, when you can.
You likely already know the above but perhaps other readers may not, like Darebee and McGyver
Just kidding guys, I read all your post and you are way ahead of me in machining experience.
Ken