Excellent Choice of Blank to Hardware, Nice Pen, Great Finish.
It takes a while to develop enough confidence, and familiarity with the tools, to turn the Blank a little closer to the Diameter of the Bushes.
Purchased Blanks do cost quite a few Dollars, and you don't want to spoil or damage them.
To get that necessary confidence, I would suggest that you take some Firewood, cut it into Pen Blank size, drill the diameter of your mandrel shaft, then Turn the Firewood Blank, to the diameter of the Slimline Bushes, that were supplied with your mandrel, try keeping the whole of the Blank to the same diameter.
Just keep using fire wood, until you build up your Confidence, and Muscle Memory, it will not take too long and you will know when you have the confidence, to spread your wings.
I look forward to seeing more of your Pens.
It’s good advice, personally I don’t intend to do slimlines anymore, it’s a long of work, no tolerance and you end up with... a pen. There’s barely any wood to see so no matter how pretty it’s not exciting. If you want to sell it there’s people out there selling them for $8 so no money in them.