I've become aware of two major problems with my current design, one fundamental and one simple hardware.
The fundamental one first: as you've probably gathered from the title, I'll be switching the I/O protocol from I2C to I2S. I'd been under the impression that the I2S-related pins on Feather boards weren't broken out, and only learned in yesterday's Hackchat that it is possible to attach I2S to a Feather (as some people clearly have done). I'm still trying to figure out which pins to use, as help in that regard is fairly sparse, but once I figure it out I'll be switching not only this board but also the two other audio-related Feathers (the TunerWing and the EchoWing).
The other problem is with the connector blocks for IN and OUT. It turns out that the footprints for the TE Connectivity ones I'd chosen are, relative to the board itself, huge -- and of course they don't fit the spots that I'd set aside for them. I'm going to switch to something else -- probably Molex, though I'm still wishing to put something there that can accommodate bare wires.
On a related note, I'm switching to Kicad. Once I worked out how to install a Template for FeatherWing, I found it fairly easy to use -- not much harder than Fritzing -- and quite flexible. The files I'll be uploading in the future will be from Kicad, replacing the Fritzing ones I've been using.