A relatively easy - but definitely slow - way to improve thin, stony soil is to spread a thick layer of nutritious mulch over it (we used composted mushroom manure, but any composted manure will probably do), cover it with membrane, and leave it alone for 3-4 seasons. When you come to plant, you should have a nice, soft, worm-rich soil under the membrane.
When we did this we covered the membrane with bark chippings, and planted through it. The bark chippings looked much nicer than bare membrane and prevented it from blowing away. If your garden is particularly windy Iwould also weigh it down with bricks or pots, or peg it down with tall stakes. Not tent pegs - you really don't want to lose them, only to find them years later by damaging an expensive spade or mower.
To plant through the membrane, cut a cross in it to access the soil. This helps keeps moisture in the soil, reducing watering, as well as reducing the amount of weeds that would otherwise colonise your lovely new soil.
I would add aucuba, aka Japanese spotted laurel, to the shrubs suggested. It's pretty much bomb-proof, will grow anywhere yet is not invasive, and provides year-round colour. If you get a male plant and plant it within a metre or two of female aucubas, you will get lovely red berries on the female plants.
I'd also suggest spring bulbs. Buy a couple of hundred in the autumn and spend a weekend or two planting them in random groups everywhere. (Not where you might plant food, though. You would not want to confuse them when harvesting, as many spring bulbs are toxic.) It's relatively little effort for massively cheering reward in early spring when there is otherwise very little colour in the garden.
And don't worry too much about detailed planning. If you're not landscaping or planting trees that will take decades to mature, you can adapt and change over the years.