I think these are great ideas. It does not appear possible to achieve sustainable farming in the early game presently.
Though, an alternative to adding grass would be to adjust the fuel requirements. You could have some less desirable plants net positive soil and other more desirable be unsustainable, forcing a more considered combination ("rotation") of plants. E.g. I believe tomatoes presently convert to something like a sixth of the soil needed for its production. I could imagine e.g. lettuce converting to a fair amount of soil vs its saturation.
I'm not sure the historical fuel value for land fills are that important; and you must consider dropping it as an alternative altogether? (soil e.g. crucially containing suitable minerals)