I’m probably in a similar boat thanks to 4x NAS drives (in 2x mirror vdevs so essentially half as power efficient too). I wonder if using an SSD or two for things like caches would help with power draw since you could defer disk usage for longer by relying on a more efficient cache.

SnapRAID is also an option. One benefit is that multiple disks don’t need to be spinning at once to access data. Downside is that your parity isn’t calculated in real time so less data redundancy.