I built my 'Big Bertha' boiler and modeled the firebox after the US Stove firebox..
So I had all the problems associated with a large firebox, that was too shallow, and had 'V' shaped tapered firebrick at the bottom 'funneling' the ash and fire down to a central row of grates...
I ended up making a firebox reducer... a moveable wall blocking off the back half of the firebox,, with a steel plate over the grate, blocking the airflow up through the grate in the unused portion of the firebox.
As you mentioned fill the firebox, 'no excuses'
That is critical.. It is a hard transition for wood burners, people think they will have too hot of a fire. But with coal, the deeper the fire, the better the control of the heat output, the less ash, and the longer the burn.