I replace dampers annually, head caps semi-annually. Yes, they will usually last longer, but why wait for something to fail. It's best to replace all dampers at once for even ink flow through all. I can park the printer for a week, or run it 10 hours daily, and don't get drop outs.
As far as the banding on the Cyan goes, nothing in the test print shows anything that should cause it, all nozzles are firing. Have you ever done a media correction? If not, that's where I would start. Go into the setup menu and find FEED COMP and run the test print. If the test bars are apart lower the value, if they overlap increase the value. Keep doing it till the two printed strips look like one, no gap, no overlap. It's really simple, and corrects most banding problems that don't seem to have any other obvious cause. It's not something you can screw up, you can always set the value back to zero, or whatever value it's currently at if it makes anything worse. Hope that helps.