Best method to determine temperature while heating sugar

IR thermometers are not accurate enough for sugar work. They make some assumptions (like reflectivity of the surface) which are not exactly met in real life.

My way of making small amounts of sugar syrup is to use a small pot with a long handle. I have a 12 cm stainless steel one with a long handle that's very comfortable, and similar vessels exist in even smaller sizes. This has the added advantage of not having too thin a layer of syrup, which can overheat quickly.

If your amount is too small to fill even a tiny pot, you might consider simply making more than you need. Sugar is cheap (OK, I know honey isn't always, but I doubt that you need the best honey if you are heating it) and if you don't feel good at the thought of throwing away food, simply making enough additional syrup to have for two glasses of lemonade will probably be enough to work with a 8-cm pot.

I use IR thermometers all the time , the are very accurate. I use them for fudge, aquariums, ponds, inside and outside temperatures ( you need an appropriate solid target) skin temperature, etc.. I have checked them against electronic immersion thermometers, all good. There are different temperatures in boiling sugar- foam, edge of pan , etc, use some judgement.Maybe because I got experience using them as a boy ,measuring furnace temperatures to 2000 F+ ,I learned everything is not always the same temperature in a given location.

You can use the drop/"ball" method. Drop some of the mixture into a small dish of cold water which will cause it to cool quickly, then feel how soft/firm the drop is. Continue cooking until you get a drop that's firm enough.

If you're familiar with the recipe you can use pretty small drops and usually only have to do 3-4 drops so there's not much waste.

(I do agree with another answer that using a narrow enough pot so that your mixture is deep enough produces the best cooking results for cooked sugar in general, especially in terms of temperature control.)

A probe thermometer should work fine, but you really want to use an electronic probe, not a mechanical one. A mechanical thermometer will sense the average temperature along part of its length, and so it needs to be immersed deeply in the liquid. Even with an electronic probe, use the tilting method to get it as deep as possible. If the probe isn't remaining in the syrup the whole time, wait several extra seconds for the temperature reading to stabilize.

I have not tried what you do. But there are a number of multi-cookers on the market. You put whatever you want in the pot and you can set target temperature + time too cook on it.

Just make sure to take a model that allows you to manually set the time and temperature. As well see what temperatures are supported. Whether you can only move with 10? increments, min/max temp, etc.