iterate through 8 thermocouple sensors (MAX31856)
You CAN store them in an array, but NOT in an array of pointers to characters. That is just the wrong type. Try
Adafruit_MAX31856 thermocouples[] = {TCouple1, ...}

or even ditch the first declarations and use
Adafruit_MAX31856 thermocouples[] = {
    Adafruit_MAX31856(2,11,12,13),
    Adafruit_MAX31856(3,11,12,13),
    Adafruit_MAX31856(4,11,12,13),
    ...
};

I don't have that library installed, but I tried compiling this pattern with the Bounce2 library and it works. That looks like this:
Bounce buttons[] = {
    Bounce(),
    Bounce(),
    Bounce()
};