Sale of arable land / land with development potential to private individuals instead of the municipality

Because it sounded as if land automatically developed would come out of the reallocation magic hat (?)

Yes, it does. Only the time at which the allocated developed building land is actually developed on site and can therefore be built on is uncertain.

Aha. A friend of mine owned such plots last year that had already been subdivided (in the sense of replanned, cut off with ample traffic area and then renumbered), but he would still have had to organize and pay for the development himself.

but he should have organized and paid for the development himself.

I do not know any land readjustment procedure where this was done, but theoretically an allocation subject to development charges is also possible. However, the municipality can no longer capture any values, which is why I did not consider this option at all. If this is planned that way for the original poster, his calculation might work out, although the development costs would then only be a rough estimate by the municipality, which often turns out to be way off in public contracts.

but theoretically an allocation subject to development contribution is also possible.

In the case of my friend, the municipality seems to not care at all whether anyone actually follows up with development after the land readjustment.