Costs for road development - City wants to open up building plots

Maybe the city will also agree that the vertical field path, once dissolved, will not be built into a street and will be reassigned to the TE and their neighbor. Then the street problem would no longer be relevant apart from the existing one. In any case, negotiate with the city; usually, something can be arranged.

with a registered right of use in the land register for over 100 years,

More than 99 years won’t work, due to customary law ;-)

I keep all the meadows, but the other owners sell. The city nevertheless decides to develop the building plots. The field path becomes a road again and it borders all three of my properties.

Completely independent of when you sell and how long you leave the meadows as meadows from your point of view: as soon as they are legally building land, you are involved in the development levy. How long you wait to evaluate the building land status also by selling is irrelevant to the municipality. It seems to me you live under the illusion that as long as you let the properties be grazed, no bill for the road construction would come (?)

What is this proportion based on? Adjoining length to the new road? Total property area?

Based on the area – possibly not as the only factor.

I just don’t want to sell areas now and then pay most of it back in a few years if the city wants to claim any development or road construction costs afterwards.

A disproportion between the profit from selling the first meadow and the development levy for the second would only result if you sold the expectation land at meadow price. Road construction costs are to be billed promptly after completion. Completion can also mean a final stone when everything else has been forgotten for twenty-nine years. Municipalities are crooks ;-)