How to change only a single value in a POST without going over each parameter with its current values?
For changing (advanced) config settings this is currently not possible (in general), as all information is stored on the device itself and only cached on the Web API. So when sending only one parameter we would either have to set all other parameter according to “know values” from the cache (which could lead to issues if you very quickly change to parameters in a row) or just write that parameter (which on the other hand could lead to unexpected results if you make changes depending on a maybe not updated state in the cache).
So as you see we are doing it that way to avoid issues with some edge cases, which normally may never happen, as by providing all values you can be sure that they are set correctly after the request.
With the introduction of advanced API endpoints with webhooks (see https://developer.nuki.io/page/nuki-web-api-1-3-0/3#heading--advanced-api-integration and https://developer.nuki.io/page/nuki-web-api-webhooks-10/8) we should be able to further decrease risks through those edge-cases so feel free to add a feature request and start collecting votes for it.