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.

I am already running Snapraid and machines do spindown after sometime, but it's still around 50w.

This is the reason I use a NAS specific box. So much more power efficient

Easiest way is to use a tasmota based power plug.
They need to calibrated once,but then are pretty reliable and can be found for 15 bucks.

Nous A1T (or similar nous,but watch out for the T at the end, Z is zigbee) is popular in central europe. They are well built and cheap as fuck. But again,they need to be calibrated once which you need a steady user (e.g an old incandescent light bulb ) and a multimeter for... It's easy and only needs to be done once.

Another option are the Inter-Tech PDUs, they costs around a 100 Bucks, are fully IP, can switch channels but only measure the consumption of the whole strip.
If you have a more advanced USV they often have a total power consumption measurement.

If you want to go all in you need to look for "switched and metered" PDUs, but they are fucking expensive. The Cyberpower PDU81005 is the cheapest "good" one and is over 400 bucks here .
So.... Most people won't do that,even in a professional setting.

Old PCs often have that problem.
My "NAS" is more a full on proxmox server with an AM5 CPU, 64GB ECC and yet it halved my power consumption compared to its predecessors.