Kachelmann seems to be more media oriented. I am not aware that there is a free API offer.

It looks like Kachelmann is also integrating weather forecasts from different national weather services and the paid version from ECMWF. I am sure the "HD" forecasts mainly use DWD ICON models as well. In this case, data quality would be the same.

I also spend some time to carefully integrate local and global weather models. For solar radiation forecasts, a clear sky radiation model is used to correctly interpolate data to 1h resolution. In the end it is a tradeoff between simple API with low operation cost and data quality/amount.