Update. I'm up and running with the new power supply and have completed a parity-sync rebuilding Drive 3 to itself, and things seem back to normal.
I found a solution in ASUS's help pages here, in Step 1 C. Turns out my motherboard has 3 power connections. The 24 pin, an 8 pin and an extra 4 pin. I didn't have the 4 pin connected! Fortunately the new PSU came with a two cables that have 8 pin connectors which split into 2 x 4 pins on one end. The AX860i does not seem to have those, but maybe I lost them at some point.
The parity sync completed without errors, and hopefully I've seen my last Red X for a while.
Thanks as always for your patience and advice, I really appreciate it.
