I did almost exatly the same with my impossible mage
Though i had access to 3 kind of dragons and archdemons mid game (lvl20) and time stop (25) so i used red+emerald+black+archdemon.
For harder, anti.hero fights i did put 2 stack to reserve or even 1 to a castle and fight with only one, but for normal stacks the bigger army ensure faster victories. It does not have any advantage over "real time" time saving but it was important to me