I would opt for something like an annual rye grass as a starter seed set, this is easy to get rid of once it has done the job of establishing roots and putting some exudates in the soil.
I would next lay down as much spent coffee grounds as I could get my hands on, right over the annual rye grass seed.
This will aid in drawing earthworms to the area, provide some bacteria and fungi along with some beneficial slime molds for a soil building boost.
Next I would try to make a compost tea or even a manure tea to water the area with, this will activate the soil microbes that are present and get them to working.
Once the annual rye shows that it can grow in the plot, you can overseed with a mix of buckwheat and vetch or any combination you have or want.
As you work on this plot, the more biological activators you can add, the faster the soil will remediate from the herbicides that came with the straw.
Things that help with this, besides the obvious mushroom spawn/hyphae are minerals applied in small quantities several times (around once a week to start then cut back to once a month).
Even things like DE, unbleached flour(both are used as light dustings) will bring items to the party that the soil microbes like to eat.