Any advice on preventing black spot or plants withering
I also love zinnias, and though sometimes they develop spots on the leaves, they still bloom beautifully. Here's a good read on those spots you may see: https://extension.umd.edu/ipm/commercial-greenhouse-production/tale-two-leaf-spots-zinnia
HEre is a good post on hoew to grow them https://www.almanac.com/plant/zinnias
Withering might mean not enough water!
I don't like to use chemicals when it can be avoided, so when my zinnias get any kind of fungus, I simply strip those leaves off the plant. You can help to avoid fungus on your zinnias by making sure there's plenty of air flow around them. Also you can just choose to look at the flowers and not the ugly leaves. Good luck and happy gardening!