I decided to rework the I2C version of the board, as though I was putting it together from scratch. I started with the ADC and DAC sitting right next to the I2C ports, then I set up the mike and speaker connectors to the other end of the board with their respective chips nearby. Then I put in the passive components one at a time, using the shortest routes possible. I think it came out fairly well. Certainly there are fewer twists and turns around the board (though about twice as many vias),
I'm calling it the "semifinal" because it has yet to be tested; passive components may need to be added, removed, or changed in value, and other changes may be needed.
Heck, if I was confident that the CoolAudio V1000 was I2C-compatible, I would've added it. I'm pretty sure there'd be room. (Not 100%, but fairly sure.)
We'll just see what develops from here.