Thank you for the suggestion, but I already tried replicating those steps. One problem is that NUC6 BIOS setuip look totally different from the NUC10 BIOS. I suspect this is because NUC6 uses Visual BIOS whereas NUC10 uses Aptio V BIOS. Also, when I downgraded to 0067, I found what apparently is the same as this Legacy S3 Standby, but it’s disappeared from 0073; anyway, it didn’t help when tried different settings of this in 0067. All other settings are as per Stephen_Mills’ setup in that thread.
I will now try with a SATA II SSD I have lying around, to see if this is only M.2 related. Like I said, I can Legacy Boot from a USB drive, so the only issue is with Legacy Boot not seeing the M.2 drive.