[Newbie] Help with DAC

Hi.

I am very new to Volumio. I’ve been using it for almost two years but I just have it on a Rasp2B connected throught the 3.5 output to my house multiroom system. It is working well, no problems here. The sound quality is ok for that purpose.

I also have a 5.1 system on my living room with nicer speakers that is powered by a Denon AVR3805. It serves me well for 5.1 movies.

My questions are:

I would like to also listen to some music on the 5.1 system. Is there any way I can have both the multiroom and Denon connected?
I would like to connect the Denon to the rasp using digital cable (coaxial or optical).
Do I need to have a DAC to listen higher quality flacs or dsd’s?
Is it possible to play 5.1 audio files on volumio (like some 5.1 super audio cds)?
Does it need a special DAC to decode 5.1 files?

I am very very confused with all the DAC types (and prices). Is there any entry DAC that can do what I need?

Thanks in advance!