Thank you, @Samuel_Tai ! The PSU (A X-400 Seasonic for which I cannot even find the purchase receipt!) is now heading off into retirement. Sadly, a titanium 400W Seasonic is not made so I had to buy a bigger one that now happily does its duty.
The NAS rebooted without any fuss, the datasets are happy, and the only ??? is whether I should do something with this warning or ignore it:
Warning
IPMI System Event Log low space left: 40680 bytes free (24822 bytes used).
2026-08-03 22:58:21 (America/New_York)
I presume this is a leftover from when the IPMI was spamming the log with 4.71V on the +5V bus warnings. Question is: will it delete itself, do I need to give it a nudge, or should I just dismiss it?