Coals are important source of coal coke, having 3 stacks of them and using them for engines is just wasteful, as you have also mentioned having a tree farm is efficient except from the fact that it is also time consuming. The main idea here is, cool down-heat up cycles of steam engines and boilers are downward fuel consuming. Alternative solution could be adding two sets of hoppers and making mine runs short yet efficient ones so you will never have to turn off your steam engine, and alternatively you can use stirling engines with scaffolds twice as efficient as coal and there is no heating cycle involved.