Any advice or suggestions on what to use? 

K type?

I can tell you what not to use. One that has a red line at 2000° F and goes to 2200°, mine melted over 2000°.

There is no thermocouple that can stand up to the highest temperatures gas forges can achieve now. There are plenty that can stand up to the highest temperature you have any use for. Thermometer prices go up with temperature ranges; they get expensive pretty quick.

Optical Pyrometer?  I had a friend whose would read out 2300 degF when he was checking if his propane forge was ready for forge welding.

They use to be the gold standard for evaluation of torch flames. Whether LASER equipment is better or not I haven't a clue.