Dismantling of slurry cellar / Preparation for lawn planting

Hello everyone. The following question(s) are on our minds: We have converted an old barn into a residential house or are still in the process. Now, in front of the house, there are 2 manure pits from an old barn (which has been completely demolished except for the pits). A garden is supposed to be created there next year. Mostly lawn. The manure pits will now be professionally dismantled by a demolition company in 3 weeks and everything will be disposed of. The resulting pits are to be filled with topsoil. The remaining area where the barn was demolished is also too deep, so this area should also be filled with topsoil.

Now a lot of weeds have already grown in this area. Would you remove the first layer with the weeds or treat them with appropriate means before the new layer of topsoil is applied? Then we have a large area where only weeds are currently growing. The same applies here. In total, I think it is about 500 m². Before we make the garden next year, as mentioned, "temporary" lawn is to be planted there. How would you proceed? Does a weed control fabric make sense for this area?

I look forward to your answers!

I had rented a tiller back then and turned the soil once. Beforehand, the weeds were trimmed with a brush cutter.

Then rake once, collect, and then put down grass.

Thank you for the response. Since we have larger agricultural machines available, we could solve it differently. Does all the weeds not just come back then? In your opinion, is it not necessary to remove 30 cm first and then refill with topsoil?

How much topsoil do you actually want to fill in? Normally, you don't fill pits with it because a) it's too expensive and b) it's not load-bearing. That means it compacts.

On the large area maybe 30 cm - 50 cm. The pits are of course an argument. Should we better fill the pits with filling sand? We get both topsoil and filling sand for free. From the side, it doesn't matter.

Mineral mixture and compact every 25 to 30 cm. Finally topsoil.

Wanted to initially fill the slurry pit 3m deep with excavation. But it subsides and eventually the garden becomes uneven.