There is a feature request open for that one so if you have any of your 6 votes unallocated you might like to vote for it here - Option to convert downloaded files [done]
I am only interested in Roon ARC for offline listening so for me this one is the show-stopper missing feature right now since download my entire local library to my phone.
I have a relatively small local library by the standards of many people here, only about 700 albums. Compressed to 256kbs AAC or Opus, or even 320kbs AAC which is what I currently use when downloading my library to Apple Music on my iPhone, it is entirely practical to put everything I have on my phone even though currently I still choose the lowest storage capacity 128GB phone when I upgrade. (I don’t do photos or video so music is the only think on my phone that really uses any significant amount of storage.)