I don't think it was this, considering that the drives have just been cleared, formatted and are sitting empty.
But as a test, I disabled all my containers & VMs, spun down all the drives and left it to idle.
After a few minutes, my SAS drives spun up again, even before the Parity or Appdata drives had done so.
Having said that, all disks in the array have now become active as I write this.
I've attached my diagnostics file below as requested.
EDIT: after playing around and deleting more plugins, I think it was the Dyanamix Autofan that was waking up the drives. Just did a search and looks like it has been encountered before, including a possible workaround. I'll keep playing around to see what works best for me. Cheers for your input though guys
tower-diagnostics-20220618-1835.zip