Hello, A few thoughts. Test for bacteria. This helps guide further actions. Your pH could be a bit higher. Is there any way to change it? Hardness should be between 60 and 120 ppm. You don’t want to go lower than that or it could damage metallic piping. Can you make provisions for flushing the system at high flow rates? This would allow you to do a lot to simply flush out that slime. What sort of anode does your heater have… magnesium or aluminum? Finally, is there a recirculation, or instant hot water line?