Nice tutorial and write up! I also am struggling to make a BLE integration (for a scale). I have asked on discord for some help as there is not a simple tutorial anywhere. Just gold-plated integrations which are hard to understand.
Sounds like you figured out enough to do a ESPHome integration. From the docs, it seems you can write a value to a specific characteristic.
https://esphome.io/components/ble_client.html