The Oppo is getting older now and DAC's have come a long way in the last few years. I'd bet a good many newer DAC's will out perform the internal one in the Oppo.
Schiit Audio, a whole host of Asian DAC's, and so many more. I'd bet any DAC above $300 will out perform the internal DAC. R2R / multi-bit is what i would look for if it was me.
The other option is a dedicated streamer that can organize your files / meta data. They can organize all your files/meta data much easier then laptops and External DAC's and players and simplifies the whole lot.