I'm so sorry about your Betta. I'm new to the forum and to fish in general. I agree that it's hard to find good websites about diseases and parasites. You end up reading the same info over and over...only a few sites have actual pictures for you to compare to your fish. I'm going through what I suspect is Velvet too and it is very hard to see. I had to try all kinds of lighting tricks and use a giant magnifying glass before I could see anything. I'm lucky that it didn't attack the gills first and that the Danios showed behavioural symptoms before it got that far.
Water quality is important but I too am tired of having people automatically blame the water. I've tested everything and my water is perfect but they still got parasites. Some of these parasites can go dormant for a long time and then pop up again. While they're dormant not much can get at them. They also can sneak in on new fish and plants (or on the snails that snuck in on the plants).
It's not a bad thing to keep a copper based med (or even Had-A-Snail, which is cheaper) around. It is a good general remedy for many parasites. Used correctly it should not harm your fish, so you can use it in emergencies even if you aren't sure which parasite you've got yet. Some people periodically put lower doses in just to keep potential parasites in check.
Karen
PS Our Betta is just "The Betta" (the one and only)
