Remove stones from the ground in the garden

A mini excavator is not an option, it will not fit in the garden (we have to go through the house to get to the garden). I think they are rubble stones (with a hammer and steel rod you can eventually break them with a lot of force, that's why I am now looking for something that could make the work easier).

Electric demolition hammer with (self-built?) 2m long chisel, possibly search for it later

We had that back then too. Before starting construction, we removed the soil with a wheel loader and then sifted it, then redistributed the sifted soil plus a bit of new soil. It was about 1000m², and two years ago we simply replaced the rest of the garden without sifting. I think you won't be happy without sifting or a complete replacement; it's really annoying if you can't break ground without having to work your way through a layer of stones.

Do you think the jackhammer can crush the stones? Or does it only press the stones into the ground?

The stones are only at a depth of 40-80cm, after that there are no more

Which chisel would you recommend? Simply a long 600 or 1000cm flat chisel?

Which chisel would you recommend? Simply a long 600 or 1000cm flat chisel?

Yes, borrow and try. That does not sound like granite stones, but rather a demolition layer that was not removed but filled in.