you should be right, some good tips HERE. But in your place i would avoid spin down, if you don’t have the total control on data access.
Reducing the number of disks is also a good point, but calc the price difference on the watt reduce gain… probably is not that worth if you plan this upgrade just for that (you will not gain any more space).
also the Xeon E5-2430L v2 is not an “idle” power saver monster, is kinda old, ok is an L but this series gain something on full use, not in idle. Most benefit will probably come with a newer architecture.
My 6gen system (look signature for detail) idle around ~40w, with a better PSU i will probably gain something more (i think you are run headless too, you didn’t mention a vga). I realize the difference from a 7gen is totally negligible, so i pointed to a cheaper and “fresh” L v5.
In my experience, SSD will consume really low energy anyway in truenas… but in case you can add only 1, and replicate nightly the data on the rust disks for an easy warm backup.