Which machine/ product would be best for melting then maintaining sugar mixture at constant temperature?
If you need to maintain a certain temperature over time, this is the domain of sous vide cookers. They are typically made for proteins, so if you cannot find one that goes to 90 C, you can use a homemade setup instead (controller + drop-in heater + small pump). It will serve you much better than any purpose-made device, unless you can find something exactly for your use case. If you go with sous vide, don't immerse the heater or pump into the syrup, use a second container (a simple pot will work).
I would be somewhat worried about the long holding though. This is simmering temperature, and you will be losing moisture all the time, changing the ratios in your recipe. Also, I assume you are not making standard hard candy, as the temperature is too low for that. If you have other ingredients, they may not take well to being kept heated constantly, for example if you are making gummy candy, gelatin will not work well (or at all) after being kept for several hours at 90 C.