To my near-complete lack of surprise, I've made a couple of significant changes to the design.

For one thing, I realized that programming the EEPROM would only take a DPDT switch (the switch and the Write Protect tab can be controlled with the same signal), so I replaced the Nexperia 74CBTLV3257 with an ON Semiconductor FSA2211. As a bonus, the new chip is also much smaller.

Then I also realized that there are too few Arduino processor boards with a Bluetooth option (see below). Most of the projects that I want to use this for involve controlling the FV-1's settings via Bluetooth using a phone app, and plugging in a Bluetooth Shield seems like just adding an extra 3/8" of thickness unnecessarily. On the other hand, not every project would call for such an app -- not enough to warrant the design time and expense of including it on the board.

Fortunately, Adafruit offers (among other things) the BLE UART Friend (Adafruit 2479), which allows me to make including the Bluetooth capability optional by putting a row for a short-pin header. I had to juggle the traces from that header to the Arduino pins a bit to allow the sample code that comes with the breakout to work while also allowing it to be mounted within the Shield's boundaries, but with a bit of component-shifting there's room for both that and even including a couple of mounting holes. And if I have a project that doesn't call for BLE or if I decide to use a processor board that already has it, not installing the Friend frees up those pins for other Shields if they're needed.

Now I have to just hope that the Shield's components aren't too tall to install the Friend. (Looking at the data sheets, I don't think they are; the crystal is the most likely offender, and I'm pretty sure it's fine.)

An aside...

I favor Adafruit's METRO M0 Express in general, but it surprises me a little that the company doesn't offer a version with BLE, especially given that they have it on multiple types of Feather boards. I have been able to find BLE-capable processor boards such as DFRobot's Bluno and Bluno M3, as well as models from HiLetgo, Emakefun, Diymore, and others that are listed on Amazon. (Personally, if I go this route, I'll probably look at the Bluno M3 first.)