1m and a bit of height difference - I would also fill in the front + the terrace at the back. Then stairs from the terrace into the garden. Since you are building with a basement, you can easily raise the ground floor to street level. At the rear sides (towards the garden) + the rest of the garden, I would leave it free to accommodate basement windows. Of course, it probably needs a small step here and there - but the slope owner has to live with that.
How big is the plot exactly? Are garages/carports allowed to be built on the boundary or not? That is not entirely insignificant, as you are aiming for 3 parking spaces.
I would wait with the neighbors' filling offers for now. You mainly need gravel or other stabilizable material. Earth is only needed if you want to spruce up the bare spots, and you probably will have enough of that yourself in the end. I would also advise against filling the entire site. First, it is questionable whether that would be permitted, and then you always have to consider that you might have to support the neighbor’s property or affect the natural water flow and whatnot. That usually causes only trouble and costs.
You can also ask what your neighbors on the left and right are planning. It is, of course, not great if everyone fills in and you are the only one left sitting in the hole. Does the development plan say anything about it?