I’m using a direct dac that supports native dsd 256 on the roon rock.However, it is downsized to dsd128.
But 256 might only be available on Mac, windows.
Will DSD256 be available via USB after upgrading Linux OS?
It will depend on what Linux kernel you end up with and if it supports native DSD256 - many don’t.
Why not look at trying something ropieee.org on a Raspberry Pi as a roon endpoint or getting a small Windows based PC that can load the needed drivers to support your DAC - Mac maybe similar issues to Rock or Linux.
ropieee is kept quite up to date, but can always check with Harry @spockfish if you particular DAC might be supported too.
Does your ROCK run on an Intel NUC?
I wonder if all Windows USB ports will support DSD512.
With Windows you have to load the drivers provided by the manufacturer.