@Dan_M Have you read this post NDS End Of Life - #252 by Mpaletta01 ?
Clearly the NDX2 → NDS using S/PDIF, which is how ND5XS2 → NDS would also function, is bettered by the SonoreUPnP → NDS configuration.
Have you tried a UPnP Bridge solution to be able to offer side-by-side comparisons to underpin your recommendations to the OP?
The OP is wondering whether a 2nd PS to the NDS, when used with the SonoreUPnP → NDS offers better value than an upgrade to an ND555, which frankly offers poorer SQ through Roon (according to many here on this forum) and is still hampered by firmware issues (well documented here also).
My recommendation based on a current NDS with 2 PSU in a SonoreUPnP → NDS configuration (also tested against a S/PDIF based input configuration in a side-by-side comparison) is to improve the quallity of the NDS processing with the 2nd PSU.
This is over an expensive upgrade to a ND555, which offers the same processing architecture, same DAC chip technology, the addition of a tiny colour screen and new remote, builtin Roon support but at a lower SQ than the UPnP input, up-to-date Tidal & Qobuz support but using on-board processing which lowers the SQ, plus where Naim can’t get the firmware working properly for over 18 months now.