My son, when he was a cello student, was always interested in playing 20th & 21st century music.
Some of the most fun 20th century student pieces we found were:
“Three Easy Pieces” by Paul Hindemith
“Sonata for Solo Cello” by George Crumb
“Cello Sonata” by Claude Debussy
“4 Short Studies for solo cello” by Bernd Alois Zimmermann
There were more, but these come to mind, all these many years later, as pieces he could work on without an ensemble. It can be hard to find teachers interested in having students play New Music but they often can be persuaded to let the student play 20th century music.