You could possibly use cement board as an alternative that may prove easier to work with.
You should put whatever you use on the inside of the legs and frame work as the retained heat will burn off the paint, etc if the insulation or board is placed on the outside.
Both mineral wool and cement board are classified as noncombustible so they won't catch fire.
With mineral wool you will have to protect it from moisture.
But you idea will work and keep more heat where you want it.
Black Walnut tapping
2021 - 3 trees 5 taps enough to boil a sample.
~ 2 acres, 35+ mature Black Walnut
2022 - 30 Southern Ontario Walnuts tapped, 69 spouts.
650 litres (143 Imp gals) on sap bags, 14 litres (3 Imp gals) walnut syrup on turkey fryer.
2023 - 35 trees tapped, 28 mech vacuum & 7 on gravity. Propane fired custom evaporator, 2'x2' divided pan, 2'x1' preheat pan.
882 litres (194 Imp gals) mechanical & gravity vac turned into 18.25 litres (4.8 Imp gals) syrup off my DIY evaporator