I think even a swale made with a spading fork (i.e., achieving its depth only due to the loosening of soil) would do some real good. P.A. Yeomans tended to control waterflow on slopes with a similar method, and seems to have gotten spectacular results.
Re-arranging biomass grown on the slope would, in my opinion, accomplish more for the labor than using a shovel alone. Windrows or bundles of weeds, staked down or shallowly buried,
should improve conditions enough to grow field crops, the residue from which is substantial enough to be a real help in getting longer-lived species established. The volume of each
root system that grows will also add to the depth you can achieve.
I think our central examples of swales are so large partly because of the organizations that built so many of them for erosion control. A system of swales will function perfectly if it can hold all runoff on the slope, and so deeper swales can be spaced much more widely. A larger number of swales is not only more fault-tolerant, it can gradually build capacity from "does some good" to "enough for a typical rainy day" to "enough for any reasonable amount of rain."
There are important functions that do require some significant depth, though. Most
permaculture books would recommend hiring an equipment operator to implement the whole plan right away, and I think that point of view is worth some careful consideration, especially because my understanding of this topic is mostly theoretical.
"the qualities of these bacteria, like the heat of the sun, electricity, or the qualities of metals, are part of the storehouse of knowledge of all men. They are manifestations of the laws of nature, free to all men and reserved exclusively to none." SCOTUS, Funk Bros. Seed Co. v. Kale Inoculant Co.