I don't keep monitors personally but from my understanding mince is no good for them. It doesn't have much nutritional value. They benefit more from eating whole prey items with all the roughage (teeth, bones, skin and fur) which means cutting up mice into appropriately sized pieces and feeding that to them as well as gutloaded insects with they are younger. I know there was a thread here a while ago about a lace monitor that developed severe MBD due to being fed nothing but chicken strips? Perhaps someone more experienced with varanus could correct/add to this.