Buy an electric motor hoe / garden tiller or rent a gasoline model

The question is whether the soil is flat enough for the requirements of the questioner. Or if there are larger chunks that he wants to break down before sowing the lawn. For that, using a motor tiller would be helpful. Then rake everything and roll it. Then sow the lawn on it (I would do that with a spreader given the size).

True^^

We here in SH also have such wide rakes, which are probably called [Rechen] more in the south, but with steel tines, not wood. You can also easily loosen and level larger pieces with these things. For sowing, take the cart, which is probably quite reasonable for an inexperienced arm throw. Although grass grows like weeds and spreads quite well by itself. If there is ever a gap, it quickly closes up.

Good morning

I had a farmer/contractor come with a rotary tiller on a tractor for about 1400m2. Tilled in different directions for just under an hour and the area was almost level and well loosened. Lightly worked over with a rake and grass seeded.
Did the whole thing at the same time with a neighbor and then rolled it.

Olli