The couple commercial induction heating shops I've been in used a lot infrared (IR) temperature sensing. An outfit the sells induction heating controls mentions both thermocouples (T/C)and IR sensing
https://ultraflexpower.com/learn-ab...ntrollers-for-induction-heating-applications/
Both T/C and IR sense surface temperature, unless the thermocouple is embedded somehow, like in a drilled hole.
The temperature at the bonding joint is not necessarily surface temperature.
Some research is necessary to determine the relationship between surface temperature during heating and the bond joint temperature. I would not expect to find that information on-line because not only it highly application specific, but likely to be considered proprietary, given the cost, time and effort to determine that relationship.
They mention (bottom of the web page) the issue with a thermocouple being affected by the magnetic fields and recommending placement outside the field, which limits the functionality of the T/C.
They fail to mention the two major deals with IR sensing
1. emissivity.
IR reads the surface temperature which emits IR energy. Different materials emit different amounts of IR - shiny metals emit only 10% of what an apple, most plastics or a painted surface emits. The IR sensor (or what reads the signal) has to be 'calibrated' for the product's emissivity, because huge errors can result, emissivity being a proportional factor.
A classic work-around is to paint the surface black in order to get near 100% IR emission.
2. target size, FOV
IR sensors have a field of view (FOV), in effect, a circle at distance from the sensor that is the surface area 'seen' by the sensor. However that FOV registers 50% of the emitted energy, the remaining 50% is outside the FOV and if there is a major difference the 50% outside the FOV will contribute to the reported temperature value.
What that means is that the FOV should be half the diameter of the object that is being viewed, with the spillover outside the FOV still being on the heated object.
Opinion
Given that there are experienced vendors dedicated to the field induction heating, I wonder if buying an induction designed to do what you need it to do, consistently and reliably, might not be a better approach than re-inventing the wheel.