Requesting suggestions for improving a local music sync tool

Hi all,

For improving my rust skills, I created a small tool Muco for synchronizing my music collection across multiple mass storage devices which would be plugged in to car stereo, bluetooth speaker, etc.

I have a central repository which may contains songs in lossless format, but most equipment only support mp3. Muco converts to the appropriate format and synchronizes the library to external devices automatically.

I am pretty sure there is quite a lot of scope for improvement. I'd like to know how can I make my code more elegant and performant.

At the moment Muco does not cache the converted files, and I want to add this feature. What strategies are the best ?

I will likely make a GUI frontend for this. Are there any best practices or guidelines for making an API ?