To isolate 2 PCbs...or not
The short answer is, "maybe".

On the one hand, you may well get rid of much of your SBC noise (or maybe not, unless you learn about control of ground currents, noise filtering, etc), but you will then have to contend with switching noise from your DC-DC converter. In general, the more compact the converter, for a given power capability, the less filtering it incorporates and the more you have to worry doing the filtering yourself.

If you're still having trouble, be aware that you can use high-speed optocouplers for most i2s systems as long as the setup is strictly SBC master/DAC slave, and in principle almost completely eliminate noise from the SBC, although you'll then have to worry about radiated noise, conducted noise, ground coupling, etc.