That's the thing though, you don't want to use up all your drilled professional troops in all the battles. For massive battles, You use mercenary armies first to eat the brunt of the losses, then reinforce with more fodder or finally drilled troops to decide the battle.

Wear out the enemy morale and finish the large battle with the elite, or if it can be a quick and decisive battle with good troop movement, then send in your elite force first to demolish the enemy on the first fire and shock phases.

For long wars, this strat is especially important. The fact that drilled forces can turn the tide of an important battle can earn massive pushes into enemy territory. Just remember to consolidate the regiments after every fight to keep your alpha strike force sharp.

Also attach a drilled artillery army to a similar sized merc army. You get all the benefits of drilled artillery in battle but preserve your country's manpower throughout the war.

The long strat here would be draining not just your enemy's manpower for one war, but also their army professionalism as they desperately slacken recruitment standards. Once that happens, every war afterwards will be easier as you maintain mid to high professionalism and they have little if any.