DSD support in Linux happens at the OS level, in your case the OS running on the SotM. It is there that support for native DSD needs to be patched per device. Which is why I mentioned contacting SotM directly.
Roon gets the audio device information from the OS and the device. Roon then presents the audio setup options based on what it received back from those device capabilities queries.
In the case of the SotM directly connected to the LKS via USB, I know that Amanero is on the list, but, I would still suggest contacting SotM directly and verify that your specific DAC will work. Keep in mind that the Amanero chipset has issues with Linux at Native DSD 512. Which is why in the SotM manual they list native DSD 512 for Windows. Bolded section mine:
The machine uses the Italian Amanero USB module, supports PCM and DSD decoding in USB access mode, PCM supports 44.1K ~ 384K various rate, DSD support DSD64, DSD128, DSD256, DSD512 (Windows).
In the case of the SotM to the Gustard to the LKS, in that case the SotM would need to have been patched with ES8620 Support (the Gustard’s USB chip) into their OS. Again, I would contact SotM to verify as I did not see it on the list I linked for you previously.
There had been an earlier thread on the Gustard, see:
Also, I would contact Gustard and see if their IIS pinouts work with the LKS. Reviewing the manuals, I noticed that the pinouts for the HDMI IIS connection are different. See clips from both manuals:
For example, notice that on the LKS, the mode information DSD or PCM is on Pin 14 (red circle) while on the Gustard it is on Pin 15. I would contact both companies for clarification to see if this and the other differences pose an issue.