If you like the way the OPPO UDP-205 DAC sounds, and you're not looking for a change, I wouldn't recommend trying too hard to better it. Yes, there are people who don't like the way it sounds and so either mod it or would prefer a different DAC. Or prefer the sound of a different DAC more, while still liking the OPPO. But you should figure out that part for yourself by listening to some things.
For connecting a USB hard disk directly, it needs to be FAT, FAT32, exFAT, or NTFS. You can check what format your hard disk is in by using the Disk Utility application on your Mac. Alternatively, if you have network connectivity for the UDP-205, you could ask your Mac to export the hard disk as a file share. Or consider using a Dante network connection to send optical over Ethernet if you think the Sonos Connect digital transfer and output isn't working well for you.