Single-family house, gentle northern slope, rough planning/orientation

What options are there to obtain the contact details of neighbors for completely undeveloped plots?

If you are already listed in the land register, you can inquire about the direct neighbors at the cadastre office. Whether this is possible by phone depends on how credibly you can identify yourself as the owner. In person with an ID card should not be a problem. If you are not yet the owner and all plots belong to one owner, e.g. the municipality, and you are very likely to become the owner, your reasons to prove the legitimate interest should be sufficient to obtain information.

How or when is an exact terrain profile created?

This cannot be answered in general terms. For the development plan, a terrain survey must have been conducted. If the terrain has not changed since then, you might get the elevation data from the municipality or they will tell you who conducted the survey. Hopefully, they will provide the raw data (elevation points) free of charge; you will have to commission and pay for a profile yourself.

What options are there to get the contact details of neighbors on completely undeveloped plots?

Do it like Ina Deter: "I spray it on every wall, this country needs new neighbors" Stick a stake in your property and staple a sign to it; hang it on the bulletin board at Edeka; write it in the cheese leaflet under "Club News": The Whats Sepp group "Anonymous Builders in the Krötensumpf II Development Area" by Simon91 meets every Thursday evening online and once a month at Jupp’s snack bar. Most people of building age regularly frequent the marketplaces of cat content aka "social media".

Does Massa Haus even build individually?

Since we are tied to Massa Haus

Does Massa Haus even build custom?

"Tied" means "purchase contract signed" or "plot with developer binding"? – I think the individuality lies in the color the builder paints.