My experience with optical pyrometers is using the commercial ones made by companies like Land Pyrometers. They are very expensive and probably well outside the budget of most hobby people. We only tended to use them for very high temperatures when using thermocouples would be a problem. If you are annealing and tempering then you are going to be at the low end of the operating range of an optical pyrometer. Yes you can get inexpensive ones that work at temperatures close to room temperature but I don't think these would be any good for temperatures up to 600 deg C.
Emissivity is a major consideration. Without correct calibration you could be way out. Base metal thermocouples do have their own problems but on a price performance ratio they are the best choice for temperatures up to 1000 deg C anf up to 1200 deg C for short term work.
The idea of using an optical pyrometer is nice, you can aim it at the work to get its temperature rather than just measuring the overall furnace temperature. Alternatively fasten a thermocouple to the work. You can create cascade control systems so that the furnace is itself is controlled off the work thermocouple but its probably getting a bit over complicated for the sorts of heat treatment we do.
My advice is stick with thermocouples, they are cheap, reliable and easily available.
Peter