Hallo Matthias,
The good news: it is possible to play multichannel (mch) music with Roon on a AV receiver.
The mixed news: According to the Roon knowledge base multichannel files in flac, aac and some other filetypes are supported and devices supporting ALAC are able to get this out. Unfortunately I already have an open ticked regarding mch AAC files not recognised by Roon.
Also not all devices support multichannel output.
The bad news: an Apple TV which is perfectly able to output mch is not supported as Roon device. Also the Raspberry PI using the ALSA drivers does not support 24bit and higher sample rates via HDMI without a custom compiled kernel.
After quite some testing I ended up with a NUC (or in my case a Lenovo m90n). These devices can run a Roon client or server via ROCK or WIN10 using hdmi to connect to a AV receiver.
This works perfecrtly with mch flac files and hopefully also with aac files after roon solves my support ticket (see above)
Using a Synology as data store for mch files is not a problem at all. However if you use a NUC as roon server you will probably use an attached storage there for your files.
one last remark. If you play mch files you need to ensure that no DSP is in the audio chain. Roon always converts to 2ch for DSP filtering.
hope this helps