The most challenging part will be to properly measure the thermocouple voltage of the T12 type soldering tips (like the ones for Hakko FX-951 solder stations).
The black handle in the middle is Bakon LF005 while the blue one is a Hakko FM-2028 clone.
T12 is a monoblock design, which means the heating element and the temperature sensor are coupled in series, and the different metals that are making the tip itself of a T12 are forming a thermocouple.
There is no separate connection wire to the thermocouple alone, so the only way to read it's voltage is to turn off the heating power, then to connect the heating wires to a very sensitive and very low thermal drift amplifier, in order to measure a few mV produced by the thermocouple.
For the T12 type, that will be to measure about 4mV DC, with \$\mu\$V accuracy, while on average the same wires are powered by 24V AC.
Some solder stations are using 24V DC, and PWM control to regulate the temperature. The plan for this solder station is to use 50 Hz AC at 24V RMS, and to turn off some of the semi-alternances of the 24V AC in order to control the temperature.
The advantage of using AC will be to have no PWM, so no spurious RF (also some sources recommend AC powering, and not DC, for a prolonged life of the heating element, not sure if this is true or not).
For the series ensemble of AC powered heating element + thermoucouple, the only certain moment to measure the thermocouple voltage will be during the zero crossing of the AC, so once at 10 ms, at most.
However, during the zero crossings, the 24V AC that feeds the heating element will override the 4mV produced by the thermocouple in only a couple of microseconds.
Looks like a difficult task to measure 4mV DC while 67882mV
of peak to peak AC is powering the same circuit. Some tricks will be required.
On top of that, the other side of the T12 soldering cartridge can heat up to about 50*C (120*F). When compared with the room temperature (about 24*C/75*F) this heating difference is far from negligible. Ideally, this temperature will need to be measured, too, and taken into account for properly cold junction compensation. Not sure what tricks to use about this one either.