My initial work uses the following hardware

- Adafruit Feather nRF52840 Express (https://www.adafruit.com/product/4062)

-- is discoverable by BlueTooth

-- has a Stemma connector, which I eventually abandoned

-- can connect with a battery

-- can recharge that battery

- Adafruit Grayscale 1.5" 128x128 OLED Graphic Display (https://www.adafruit.com/product/4741)

-- good screen size

-- Stemma connector

- Lithium Ion Polymer Battery - 3.7v 500mAh (https://www.adafruit.com/product/1578)

-- about the size of a US quarter, about 4mm thick, which is a decent small size

-- pretty good capacity

-- connects easily to the Feather board

I plan to try to either use a single board for bluetooth and magnetometer functionality, or get two smaller boards if the combined board has too many extras.

This board has both, but might end up having too much extra stuff (and it's $15 more than the other Feather): Adafruit Feather nRF52840 Sense (https://www.adafruit.com/product/4516)