At best you’re going to get anecdotal answers to this. For the real answer you’d need to have a friend who’s an engineer at Western Digital.
Keep it cool, keep it well-ventilated, feed it clean power, back up your data after any significant changes, and just keep it turned on.
Mine is SSD and always stays on. If it were spinning, it would still always stay on. I doubt the predictability of MTBF is sufficient to say whether on or standby is going to make a meaningful difference, so just save yourself the trouble.