After this morning's discovery, I found a resource for the Si4734, which I think is an improvement. I built (in Fritzing) an alternate version of the board, and with the I2C interface it uses only the chip, a crystal, and 6 capacitors, plus I feel much more confident that it's hooked up correctly (the main exception is the crystal, which is probably unnecessary anyway; I may remove it). I even was able to add a Qwiic connector in the bargain!trivial.
I'm debating whether it'd be worth the trouble to figure out the optional I2S output from pins 1-3. The schematic given on the Si4734 data sheet call for 3 resistors and a capacitor for that option -- meaning I'd have to migrate just about everything toward the east -- and I'm not sure whether I'd want to send it to a separate jack (probably meaning bye-bye Qwiic connector) or to some of the Feather's Digital I/O pins.
Right now I'm leaning toward just letting the board sit as-is, and making this the primary effort, but I'm open to other ideas.