"I think there should be a minimum amount of songs downloaded per offender estabished, before they are allowed to prosecute."

There is a minimum amount already: one.

"If they put more time into investigating the more active offenders, ..."

So is your complaint with the level of the penalties, or that they're not being universally enforced? If everyone who illegally downloaded content was subject to the exact same penalties as those you described, would you still consider the severity to be 'harsh'?