Example of robustly subscribing to BLE gatt characteristicvaluechanged

This is my code so far. I'm struggling to understand how to not lose context through the promises chain - the callback I register for characteristicvaluechanged has no idea which BLE device sent an update!

function connect() {
  nrfbusy = true;
  NRF.requestDevice({ timeout:4000,
                      active:true,
                      filters: [{ namePrefix: NAME_PREFIX }] }).then(function(device) {
  console.log("Found");
  show(0b10);
  return device.gatt.connect({minInterval:7.5, 
                              maxInterval:7.5});
  }).then(function(gatt) {
  console.log("Connected", gatt);
  show(0b100);
  return gatt.getPrimaryService(PRIMARY_SERVICE);
}).then(function(service) {
  show(0b1000);
  console.log("Got Service");
  return service.getCharacteristic(READ_CHARACTERISTIC);
}).then(function (characteristic) {
  show(0b100000);
  console.log("Got Characteristic");
  device = 'button1'; // how can we find this from gatt, a few promises ago?
  characteristic.on('characteristicvaluechanged', function(event) {
    parseMessage(device, event.target.value.buffer);
  });
  return characteristic.startNotifications();
}).then(function() {
    nrfbusy = false;
}).catch((error)=>{
    nrfbusy = false;
    console.log("Error: " + error);
    show("1   1\n"+
        " 1 1 \n"+
        "  1  \n"+
        " 1 1 \n"+
        "1   1\n");
});
}

Additionally, when I try to connect to a second BLE device at the same time (e.g, run that connect() twice) I get ERR 0x12 (CONN_COUNT), so maybe the nrf52833 in the Microbit v2 can only connect to a single device at a time? Although https://devzone.nordicsemi.com/f/nordic-q-a/35158/how-many-connections-nrf52840-nrf52832-supports-as-peripheral suggests 20.