Changing the holder design to the shaft-rotating one from the previous shaft-fixed one.
Works good!
I'm going to mass-produce this.
During the iteration time, prints got a lot of stringing.
It happened much when the resolution was too high, so I lowered it to 0.20 mm (I selected the 0.20mm QUALITY option on PrusaSlicer-2.3.3 at last).
It still had some striging; I heated the prints using a heat gun.
It worked well.
Middle: before heated; Right: after heated