If Roon’s “DSP Volume” is available in Device Setup then maybe you can get volume control – but you have to enable the sample rate conversion and native DSD processing AFAIK. Else you have to use bit perfect playback (Fixed Volume, no auto volume leveling).
PS: Always enable Headroom Management and the Clipping Indicator too if you use the DSP Engine (you can set the headroom to 0 dB as a starting point).
To my knowledge SACD player is not legally allowed to output DSD stream to SPDIF. Besides not everyone supports DoP over SPDIF (although our products do).
Some SACD player optionally converts that to PCM to output to SPDIF.
What you need to do is to rip the SACD to .DSF files for Roon to play them. The ripping process requires very specific hardware though.
About your original question on digital volume with native DSD, the simple answer is that it is impossible without conversion to PCM. (Ignoring additional software or how hardware DAC chips do that, etc.)
great machines, OPPOs. I still have an old BDP-93 at the center of my Home Teather system and when I started looking for a newer model, I found out the company closed the BDP production (and moved all on mobiles market)… and second-hand prices started to soar… too bad