So I just recalled I had a DC-capable clamp meter, and I was able to isolate the disk on the last stretch of a power connector chain.
I tried running with both the power management set to "Level 128 - Minimum power usage without Standby (no spindown)" and Acoustics Managment to "Maximum", as well as "Disabled" for both. Didn't make a noticeable difference on power draw, at most 0.1A more during writes, primarily just noise.
Idle
5V 0.27A
12V 0.40A
Write
5V 0.53A
12V 0.50A
So that's just over 6W idle and 8.7W under load. I'm surprised about the high idle draw, both due to being significantly higher than the specs[1] and due to them running so cool. Did they spin down the disks to get the 4W figure perhaps? I did check my clamp meter against my electronic load and it reads pretty accurate.
I didn't manage to test reads properly, since the disks are part of a ZFS pool and it spreads the reads all over, along with aggressive caching.
[1]: https://toshiba.semicon-storage.com/us/storage/product/data-...