This topic is an old chestnut on these forums. Almost all the experts here (a group that does not include me) discourage drive standby for various reasons. Having studied it and experimented with it quite a bit (
https://forums.freenas.org/index.php?posts/166522/), I can tell you that even if you address the two issues that joeschmuck raises, you still may not get what you want.
I have a script that monitors and logs drive spin/standby status at intervals. I have two pools. One has no jails, no system dataset, no shares, and is only needed once per day, so I wanted it in standby the rest of the time. For a while, things worked well and the drives spun up once per day, then went back to standby. Then there was a FreeNAS update. Ever since then, the pattern has been chaotic and apparently random. Sometimes they spin up and down many times per day. Sometimes they stay spinning indefinitely. Often, the drives in the pool don't spin up and down together, but behave individually and erratically. With this behavior, I think the guys who say you're wearing the drive out by spinning it up and down all the time are probably right. So I gave up and just set them to spin 24/7.
I don't think the development community is interested in testing and making sure this will work, looking at and controlling all of the processes that might cause drives to spin up needlessly, because as I say, drive standby is not part of the FreeNAS culture.