I'd recommend to use Arduino as a platform to practice and understand the basics but at some point the processing power and limitations of an 8-bit processor will make you hit a roadblock depending on what your goals are. On the other hand the ARM-environment is great and cheap, check out the blue pill/black pill boards. No wonder why so many digital modules have M3s and M4s inside. (Varigate, QuadLFO have ARM processors, Mutable Instuments uses them in a lot of modules, list is endless...). You can get ARM boards for 1-3$ from China (check out AliExpress).