There are circumstances when you might need to trash and rebuild the OS boot drive - so some people (including me) like to have a drive dedicated to the OS so that doesn’t affect other drive.
Using a separate partition is another approach.
If you’re buying this specifically for roon - you could consider a NUC running ROCK.
Performance wise, I would expect an internal SSD to outperform a USB connected drive at any time.
GregD however does make a very good point:
Partitioning your drive, as suggested by GregD is one option. This would allow you to rebuild the OS whenever needed, without overwriting you music. I would then create a smaller partition for the OS boot drive, and a larger partition for the data including music files.
Alternatively, adding a second smaller SSD to use as your OS boot drive is also an option.