The Jian Jian Baiyun you used and your other double-sided brush both belong to double-sided brushes, but the hardness ratio is different.
Don't be superstitious about the so-called calligraphers who use sheep's hair to gain profound skills. Wool roses in Ming and Qing dynasties had a lot to do with cursive script and epigraphy. People in the Jin and Tang Dynasties used hard brushes. Haven't you noticed that Hongyi Bizhuang, Hanbifang and Hangzhou Yi Yun Bizhuang supervised by the Calligraphy Association are all pens with deep management, good concentration and good flexibility? Who will show their skills with a weak pen?
Moreover, the double-stranded brush you use is not good at bristles. It is estimated that it is inferior nylon, which has a hard reaction and is easy to "expensive", so the quality of the pen is poor. If it is a purebred wolf with a northern tail, it is elastic and soft, not so-called bristles.
To practice regular script, the ancients thought that taking regular script as the top can not only practice bone strength, but also pay attention to the whole. I think both adult block letters and Chinese block letters can be used, as long as they are not block letters, and the beauty and China are elegant and heroic. Learning block letters for a long time will lose your heroic spirit. If you use block letters again, you will be tied behind your back. Otherwise, it will be easier to enlarge them and master the brushwork skillfully.
Misunderstanding, not all calligraphers are good at regular script, but most calligraphers are good at cursive script, so they are not good at it.
You can try Chunantang 349# to write Chinese letters. Although it is made of nylon, it is very useful. You can try. It costs about twenty yuan.