What I found so far:

• Apparently I totally ignored the teensy's need for power. It does have a ground connection, BUT
• Teensy's GND is connected to ESP'S Rx pin and
• Teensy's Rx pin is unconnected

I have some through hole schottky diodes so I'll put one between 5V and the Teensy's 5V pin, so that the Teensy can be powered via USB or an external supply. All that was needed was some magnet wire and the diode:

That seems to work. Top view:

The Teensy is currently running a crude serial passthrough sketch so that I can use it to

• program the ESP or
• debug the ESP application via serial

This is the crude serial passthrough sketch:

void setup() {
  // put your setup code here, to run once:
  Serial1.begin(115200);
  Serial.begin(115200);
}

void loop() {
  // put your main code here, to run repeatedly:
  if(Serial.available())
  {
    Serial1.write(Serial.read());
  }
  if(Serial1.available())
  {
    Serial.write(Serial1.read());
  }
}

The ESP app already connects to my WiFi using config data stored in the FFS and I'll now extend it to subscribe to MQTT topics as described in the previous log.