T.N.
It's lucky you have a never ending supply of manure! And free too!
But if you have clay soil, I'd use peat moss to break it up, in addition to the manure. In fact, you may want to try a raised bed instead, mix a good organic sandy top soil, peat moss, and manure. All of which will tend to dry out faster than your clay soil.
Also different veggies like different types of soil, and different amounts of water. So you'll just have to look at a good gardening site, or check the back of the seed packets, or plant labels when you buy for each individual veggie.
Here in upstate NY we have an even shorter growing season than you, and for leafies, lettuces, kale, etc there is never enough time for it to get big enough. Root veggies, two, like carrots, beets, ginger, potatoes, always are the 'baby' variety, tehehe.
We have fantastic luck with squashes and beans and tomatoes though. And try some herbs, too. I love having my own parsley, basil, rosemary, thyme, sage, cilantro....and get very cranky when they die back and I have to start getting them at the grocery store again.
Not sure about your sunflowers as shade theory, they take time to grow tall enough to shade, plus they may attract birds and other animals to come snack on your efforts.
I've used obelisks for sweet peas and planted the more fragile veggies behind them for a little shade and protection.
You can also buy the screening that green houses use at a garden center, or even a bed sheet (just be sure there's air circulation) draped over stakes.
You're getting me all excited about it now. It's been a BRUTAL winter, and here we STILL have 2+ feet of snow on the ground!
I want to be a farmer when I grow up too, but for now I'm just a suburban foodie!
:)