People and their gardens share space in the world with many other creatures. Some of which feed on plants we might prefer they leave alone.

Our job is to learn who can do what, aand when to catch them in the act.

Slugs, snails and many caterpillars and beetles are night-feeders. You need to go outdoors with a flashlight to see what's going on.

As for powdery mildew, it's a common summertime thing on many plants. Your job is to ask lots of questions to learn which ones have the fewest problems in your region.

Bottom line -- Gardening is a live-and-learn sort of thing.

What is your soil like? Clay? Sand? Gravel?
What does your soil smell like? A nice pleasant aroma or a putrid odor or maybe nothing at all?
What and how much life do you see in your soil?
How workable is your soil?
How much organic matter is in your soil?
How well does that soil drain?
How well does that soil retain moisture?
The leafy greens sound like they may be harrased by Flea Beetles, and the best way to combat them is with Floating Row Covers. The rest will be difficult to determine since a large number of thingies, rabbits, deer, and numerous insects, like them.

I hear you. I have cryed on several occasions after discovering the massacres that happened while i was away from the garden this summer (including a major mite infestation of my beans. The advice i get from everyone is to keep working at building up my soil and getting it healthier and to plant lots of flowers to attract beneficial insects. once you do that maybe order some beneficial insects online or buy them at a local farmers market, as for powdery mildew. it kept hitting my cucurbits. You can control it nicely with neem oil sprays.

good luck!