Build a terraced end house with an additional unit (GÜ) on your own

that sounds reasonable. Or a staircase made of pallets, not pretty... but it works for now.

I also mean: as parking spaces, a compacted area is sufficient at first so that two cars can be parked on one's own property. I like the idea of the pallet staircase, and otherwise I would also wait until at least the basement of the middle house is finished and filled up. Even if the dear wife already has garden dreams – I would find it too much of a waste of money to pay for an interim solution first and then do the final solution when it’s finally possible.

I would still have an appointment reserved with the landscaping contractor – otherwise it might become tight. Best to coordinate with the middle house owner if they have a schedule, and ask when the basement is supposed to be finished and when it will be filled, and then attach your own appointment to that.

I would also rather go with the option of stairs going down, because piling up is likely to be difficult as long as nothing is happening next door. An alternative would be a "Balcony," that is, a 1.50m high platform. But setting that up securely is more complicated and more expensive than the stairs option.

a compacted surface is initially sufficient for parking spaces so that you can park two cars on your own land,

Here clearly no. As you write it, this applies on the one hand universally – but on the other hand only for all the zero-eight-fifteens, where during the provisional period there is a bumpy track – but this one almost at the appropriate finished height – lying there. Then the cars temporarily stand 8cm of paving stone thickness plus substructure lower than finished. In this concrete case, however, at least L-beams and other protections against the viscosity of a one-and-a-half-meter-high rain-soaked mass of earth must be installed so that the cars do not slide away on an avalanche (or like over a collapsed mine). That is already a different matter – also with regard to the nonchalance potential

In a worst-case scenario, you can also park the car on the street in front or nearby until the neighbor has secured his earthworks project.

Joker. It’s not about waiting for perfect blooming, it’s about mud in front of the patio door, or in this specific case, mud on a one-and-a-half-meter slope in front of the patio door. For a detached neighboring house, this would all be a private matter, but in a terraced house it seriously disrupts the neighborhood structure.

No offense, but how many new housing developments have you already lived in?
I can tell you from experience that people like for everyone to build at the same time and then immediately work on the outdoor facilities, but no one can guarantee that.

For example, we ourselves had to wait for an adjacent neighbor. I could have put up the fence alone, but then I would have had to pay for it completely myself, and if he wanted to put a garage on the boundary, we would probably have had to redo it. So you leave it alone, or at least we do.

You can also construct a temporary path to the front door, and that has nothing to do with the boundary design. There is also nothing against completing the earthworks entirely; you just slope the last half meter lightly and wait to see how the neighbor positions himself. You can still go ahead with the terrace, etc. That’s how we did it and have seen it many times in the housing development and among friends.
As a rule, road construction also comes afterwards, but no one is deterred from creating a driveway. It gets adjusted again during or after the road construction, and then the job is done.