If your chosen shrubs are a good fit for your climate and soil, a decent amount of compost mixed into a big hole when planting should give your plants a good start. Try 8" to 18" below the
roots, and 4" to 6" around the outside.
And if your shrubs are a good choice, they will also help add organic matter (leaf litter) that can help you grow other thing later.
Your raised
berm sounds like a good starting place. You should not have nearly the drainage problems usually associated with clay soils. But it may dry out before the plants have strong roots to help them grow.
You will want to
water well during the dry season, but not every day, to encourage the plants to grow
deep roots. Especially if you have long dry periods when the top layers of clay can dry out and harden.
As far as other things you can buy -
In my area (Portland) the
city composts
yard debris, and so do several private compost companies that collect food and landscaping wastes. You can get a dump-truck-load of compost delivered, delivery costs but the total price is much less per cubic foot and you don't need to load it into your car yourself. You can get it dumped somewhere near the driveway where you want to garden in future, or just in the street and move it where you want it. Ask around for prices at landscaping companies or farm supply places.
You can also add perlite for drainage, it will not compact down like sand, but it is more costly. Usually only used for potting soils, as it's spendy and organic matter does the same job only better. Don't buy less than 2 cubic feet at a time, to get decent prices.
Charcoal can also improve clay soils tremendously. You can probably buy it from the same places above. If it's not originally intended for garden use (like you are just using the stuff from a friend's burn pile or something), make sure there wasn't anything nasty like lead paint or lighter fluid in it. Don't use briquettes, but charcoal from any type of wood or plant debris should work OK. Rinse it well, then add some fertilizer (e.g. plant food, miracle grow, or watered-down
pee) before mixing it into your soil.
You could also use woody debris if you remember to add a lot of nitrogen (pee) regularly, or grow plants that thrive in woody debris like most of our NW
Native shrubs and ferns. This is more of a long-term strategy, not a quick-fix. Lots of permies use wood chips for semi-permanent paths, they last a couple of years and give the clay soils some structure instead of being just mud.
Best advice - take a walk around and look at what the neighbors are growing. Watch to see how much work it is - are they are out tending their plants every day, or hire a gardening crew, or does their yard takes care of itself and they get to relax? Ask them about their plants. You may find an enthusiastic gardener neighbor who has filled up their own yard and would be tickled to help you start yours off on the right foot.