Im having an absolute nightmare with my garden...
I apologise in advance as my input is unlikely to improve the nightmare.
To eliminate the existing brown / green rubbish, enlist a small army of friends & family and arm each with a bin-bag and a bucket. I found bribing them with the promise of a barbeque was helpful or if that doesn't work, invite neighbours in on a reciprocal basis.
Moving in a line down the garden, have them pull the dead and green plants out of the ground, roots and all and put them in the bin-bags and pick rocks, bricks and builders material from the soil and place them in the buckets; pea-sized stones or gravel should be left in the ground.
Now have your army sweep from one side of the garden to the other, collecting as they go. Burn all the plant material by using it to start the barbeque. (The plant material, dead weeds with residual poison, can't be mulched and can't be composted).
Now is the time to dig the garden over. If you can arrange to do this manually so much the better, collecting all root and plant material as you go. Rotvating while easier and quicker will break up root and other residual plant material and spread it. As before, collect large stones and rubble dug up.
Now the bad news. As you left the weed-killer treatment so late, many of the weeds will already have set and spread seed so next spring you will undoubtedly face a brand new crop of weeds. The way this weed-killer works is that it is "systemic", travelling from point of contact on the plant to shoots, leaves and roots. Once the weed-killer touches the soil, it binds to soil constituents and breaks down to an acid and other substances and is no longer a weed-killer. Therefore seed falling on your garden or already in the ground two weeks ago is likely to sprout and produce a plant next spring, as will root material left behind by rotovating.
Next spring repeat as above without the weed-killer, this time levelling the ground and seeding as you go.