Cya is sunscreen for chlorine, and will slow the chlorine burn-off from uv. However, chlorine will still "gas off" when combining with organic contaminants, which can become trapped under the cover. Can you see a distinct color difference between the middle of the cover and the edge where it sits on the shell? That is an indication of gasses trapped under the cover. It will also bleach out your valve handles and other exposed plastics.
I have seen a few instances of rusted or pitted stainless steel, and in every case there was high sanitizer (both chlorine and bromine spas) and low ph. And I was nearly choked on fumes when I opened the cover. But the water looked great. Are you staying on top of your ph? Testing once a week and dumping in chems to correct ph is not good maintenance. Your ph is bad most of the time in that case.
I am a fan of mineral purifiers. Much like algaecide, the silver and copper interfere with the reproductive cycle (cell division) of single-celled organisms. So they slow the spread and growth of contaminants, giving your sanitizer more time to deal with them with less overall reduction in sanitizer residual. This allows you to keep lower residuals in the water, and produce fewer chloramines so need less shock. It does not eliminate the need for sanitizers. Neither does uv, or ozone, or a combination of the three.