nettles mean a very fetile soil. This garden sounds a bit like mine 18 yrs ago I dug it over. It is still uneven. Better leave this one to someone more knowlegable than me

We took over a grossly neglected garden extension many years ago, at same time as a near neighbour. We shared a small bulldozer and shunted tons of rubbish including nettles into a deep hole. Then we just let the grass grow, got a powerful hover mower, and regularly mowed, never saw another nettle after that. I'm not suggesting a bulldozer but ,providing you can get it smooth enough to mow; the regular mowing keeps most things at bay apart from buttercups and the like. Our patch was probably about 1/4 acre with our neighbour. They had no trouble either with weeds.

If you don't feel like digging the whole thing levelling and laying new turf, you can just keep cutting, as Dorjac says, with a lawnmower (preferably a hover, it'll be easier while its so uneven) - it will eventually flatten out and become more like a lawn. My father in law once kept mowing a rocky, uneven, lumpy old field and within 2 years it was good enough to use as a driving range for his golf shots, thoiugh admittedly, he was using a sit and ride mower.