Dont mess about
20lt Tea Urn - cheap from Booker
Fill partially water and turn on
When boiling, turn off and stir with long wooden handled spoon adding required amount of sugar as you go.
5 minutes later open tap to drain into 20 lt container ( food grade plastic) pre-primed with thymol.
As still warm, repeat for 2nd container but whole thing warms up in half the time.
I know the right amount of water to start with for 1:1 and 2:1 syrup so only have to make some 2 or 3 times a year, usually on a cold and miserable evening. Standing the containers out in the garage cools them quickly as well.