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Renoir and Cezanne PRO Apus can idle under 15W. I’ve built my NAS inside a Node 304 as well and use a Gigabyte A520I with a 5650G and ECC RAM because ZFS. Asus b550i works as well. As long as you activate both power savers in bios (cec/aspm/erp) your system will idle under 15W excluding other stuff. I’ve not seen under 20W with asrock boards. With a asm1166 m.2 adapter you get enough sata for the drive caddies which leaves you with 4 sata on the mainboard for cache ssds that you can mount between the caddies.

Jonsbo N3 sounds like a good fit