Beautiful pens, you must have a typo in your post though. It's hard to believe you've been at this for only a week.
The splitting may be associated with your gluing. First, make sure you are getting glue along the length of the tube. I will add glue from one end of the blank and then add glue to the tube and insert into the opposite end of the blank. Helps ensure glue along the entire length of the tube. The other issue can come from having glue in the tube. When you press the parts in, any glue in the tube will cause the tube to expand too far splitting the wood. I use modeling clay to plug the tube end before gluing. I also use a tapered reamer to clean out dried glue from the tube.
Those slimline pens are also a challenge because you have to turn the wood so very thin.
Checkout the Sierra/Gatsby/Wallstreet type kits. They can be a bit more forgiving and are economic as well. One blank makes two pens.
Danny
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