@clew @marsiposa the first thing I'd advise is to find out whether the county where you live has native plant sales, or if you can access someplace like Watershed Garden Works https://www.watershedgardenworks.com/nursery
They sell for cheaper, and often in bundles. The native plant sale near me sells bundles of 3-5 of all sorts of trees, shrubs, bulbs, seeds, herbaceous perennials, etc. For about $5-15/bundle. Good way to cover a lot of territory very quickly.
Thimbleberry is another cane berry like salmon berry that competes well with blackberry. In fact, it really works best in a situation where it's mowed back past a certain point, or it will fill up all available territory. Dewberry is the native blackberry that covers a lot of territory very quickly.
When I was trying to cover a lot of land quickly, I got a bundle of mock orange, flowering currants, native roses, native ceanothus, red twig dogwood, etc. I'd already planted quite a few huckleberry before I even knew about the sales. I took a lot of root cuttings from local areas that were being developed into housing. Mostly herbaceous perennials, but also quite a few thimbleberry, ninebark, osoberry, Oregon grape, etc. They all took beautifully despite my very rough job of digging and transplanting. It seems like taking cuttings of willow and dogwood would likely be cheap/free and easy. Bung some in the ground, some in a vase, see what survives. Hazelnut seems to grow really easily from seed.
The other consideration for me is that I had next to no infrastructure for watering, so everything I transplanted was literally just watered in once or twice, and left to fend for itself after that. Not ideal, but it was still effective.
And for the blackberry, solarizing it is good. If you can find someone with goats or sheep, they make very quick work of even the worst thickets.