Time to hot-wire my Monitor One and put an 8-pin panel-mount male pin connector on it!
I can't find what seems to be identical part as used on the Tracker One, but this one from Mouser looks like it will do the job; it just has a little bit more plastic extending beyond the metal threads.
What do you think? Will this one work, or is there a part number I can reference as used on the Tracker One?
You're not going to be able to connect the M8 temperature sensor to the M12 connector on the Monitor One. The Monitor One does not expose the UART/I2C port on either M12 connector (8 pin or 4 pin).
The I2C port is available on the expansion card header inside the Monitor One.
If you're going to connect it internally, it may be easier and less expensive to use a regular SHT31 as not only is the M8 connector expensive, the M8 temperature sensor cable also includes a 3.3V regulator because the Tracker One only has 5V power on the M8 but requires 3.3V for I2C. The Monitor One exposes 3V3 on the expansion connector.