We really got great News from Viessmann, an all New Developer Portal!
Now we "can" apply ( and pay!) for an "Advanced" plan which entitles for more API-calls per day and which also allows to use API calls that are not part of the basic plan.
In other words, all the great ideas/requests published by users in here which result in additional API-features are likely to be part of the "Advanced" plan.
I do understood clearly and will keep my mouth shut.
20€/mo for private api use seems excessive