The Full extent of the Jam is good for starting to learn basic stuff, like bullet herding etc.
Its also important to stick with one game to learn its patterns and system.
All Cave games are different and gives you practice in different ways.
For example EspRaDe has quite basic patterns and is a good practice for reaction dodging, Ketsui has very organic patterns good for practicing to see pattern movement, Dangun feveron has extremely fast bullets and so on.