Hi @joeschmuck,
thank you for the hint. However, Truenas 25.10 has introduced an issue (supposedly with temperature monitoring?) that is frequently waking up all disks - even those not being part of any pool. Hence, the solution via scripts / cron no longer works. Manually putting the disks to standby will only last for a few minutes until Truenas wakes the disks up again - without any data activity.
See here for a proposed patch to fix the issue in middlewared (which is a core component of Truenas): HDD Sleep/Spindown/Standby - #71 by ark
My production machines currently stay on 25.04 only due to this issue.