Friend - you got a lousy metal roof if it only lasts 30years. And if your roof slope was too low, then you probably needed a different type of roof, or the proper metal roof with taller seams, and proper detailing.
First, you don’t paint anything that you want to last an extended time because paint will always wear out/ugly out well before the substrate. If you want it to last the most affordable option is unpainted galvalume coated steel. It does not need to be cleaned, not any more than rain provides. If you have trees and debris that does not wash off, then yes - you should clean that away, but this is likely a symptom of your roof not being steep enough.
That roof will cost 2-3 times more than an asphalt impregnated shingle roof, and will cost less over the service life of the roof. IT COSTS LESS TO USE A GOOD ROOF.
