Hi Ko,
The 32u4 doesn’t have enough memory to run StandardFirmataBLE.
My current recommendation for a Firmata-compatible board is the Arduino Nano 33 IoT, which works well for both serial and BLE connections. Unfortunately, that one is also out of stock, at least at the Arduino store, and I don’t have an alternative to recommend. At present, I do not recommend the similar-sounding Arduino Nano 33 BLE, as we’ve had some issues with it in the DiCarlo Lab.
Sorry I can’t be more helpful!
Chris