Basement or ground slab: which is more sensible for the property situation?

I agree with you, a square floor plan would certainly work regardless and would have slight advantages regarding the modeling effort of the terrain. However, as already mentioned, I think you can't avoid a 1 m retaining wall in the northeast (which the two adjacent properties have already implemented)...

could you mark points in the draft concept where, in your opinion, sampling points (including garage/carport in the northeast) would be necessary and also how many?

I would simply select the inner circle of the building window here.

I would like to point out that the development plan explicitly refers to heavy rain events and the hillside location. The top edge of the finished floor at street level would be a bit risky for me.

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Great!! The bedroom would be suitable there, right?
We have a cathedral that never quite became a cathedral… from the church tower of which we look from the toilet. We have more of that than a strained window in the living room. Maybe that will come up again with you.

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Yes and no. It depends less on the exact elevation of the house and more on how well water can flow around the house. Even if a 10cm stream runs down the street. Anyways... to achieve this here, I would not set the top edge of the foundation slab below 156.00. You then have at least this height at the terrace as well, realistically rather 156.10m. However, the terrace extends into the original terrain area of 154.5m, from which you still have to deduct 30cm of topsoil. That makes a lot of gravel underneath, which also has to be retained. Same problem with the building’s foundation pad. It’s exactly the same crap as in our area... there the houses are at street level in front and significantly over 1m higher at the back...

Since the plot is not that big, it should be enough to roughly know where the house will be located. You have already started with that. So you should have a rough idea of where what could be. SO living rooms and terrace, NW utility rooms. I would move the house further to the NE and have the terrace in the SO and also a SW garden. That already gives two points for the drillings of the soil survey.