Electron 2G stuck cyan / green

[ScruffR]

Just a few things to start with:

@xav

// Disable GSM on boot
SYSTEM_MODE(AUTOMATIC);

The comment and instruction don't really fit together

@xav

/* Define two Adafruit_SHT31 instances */
SHT31_Wire1 sht31 = SHT31_Wire1();

You'd only need

SHT31_Wire1 sht31;  // this already calls the default constructor

@xav

bool syncTime(uint16_t timeout_ms = 1000)
{
  if (Particle.connected())
  {
    Particle.syncTime();


    // syncTime is non-blocking, so wait for a valid time to come back
    // from the server before proceeding
    uint32_t now = millis();
    uint32_t timeout = now + timeout_ms;


    while (Time.year() <= TIME_EPOCH_START && now < timeout)
    {
      Particle.process();
      delay(POST_PROCESS_DELAY_MS);
      now = millis();
    }
    return now < timeout;
  }
  return false;
}

You should not mix the datatypes here, but use uint32_t all the way through and also rearrange the calculation/value check.
The background can be found here
Millis() and rollover tutorial

Try to reduce your use of String where possible to avoid risk of heap fragmentation. As long you don't need the Steing class methods, C strings are safer.

If your LED gets stuck on a solid color, it's most likely the I2C connection hangs for some reason.
What value pull-up resistors are you using and to what voltage?