Hello Mr. Vollmar
Unfortunately, the operating instructions for your S9800THX system are only available in English on the Onkyo website.
http://www.de.onkyo.com/downloads...
You should find the information you are looking for in the "Advanced Manual" on page 26 of the document.
Since you already have enough speakers with the complete system, I would recommend a 5.1.2 system with "front-height" or "rear-height" speakers. You benefit from the height level compared to the classic 7.1 surround arrangement.
As you have correctly noticed: "Dolby Atmos" and "DTS:X" can only be selected for audio playback if "Dolby Atmos" or "DTS:X" content is also natively available on the source material!
However, all other sources such as "Stereo"/"Multiplex" or "Multi CH (= channel) - sources" can be expanded to a similar level as "Dolby Atmos" or "DTS:X". This is done by selecting "Dolby Surround" or "DTS Neural:X".
On 31.08.2017 there was a new firmware update for your device: maybe this fixes your Chromecast problem?