Sand is not the same as sand, just google Sieblinien. The question is the gradation within the individual grain groups. At the sea, unfortunately, the sand grains all have the same size.
Your "problem" will be finding the right structural engineer. Buying the plot was probably not a mistake.
The slab foundation statics cost me 1000 EUR.
Is a (assumed) 2m deep strip foundation not expensive as well? Probably rather 2.5m on the valley side.
If the statics are included in the original fixed price, can there be an additional charge due to more difficult soil statics, or is that then a cost risk for the general contractor? Sure, the soil is the builder's risk, but the associated statics as well?