I probed the traces to the vertical uSD socket and FTDI and determined that with the isolation capacitor and DTR, it’s not possible to update the bootloader from uSD/J7. Of course, Right there on the circuit diagram there is an ICSP port (H4), which is a set of pads for what I believe are 1.94mm pogo pins by the uSD slot. I wonder if there is a part number or diagram for this? It should be easy to design my own, but an engineered part might survive better if I have to reflash many of these.
Though so far I am finding that the source of power-on resets is simply having the uA meter in the power loop.