You're right. 6 spinning disks and a discrete GPU won't work with 120W. I would recommend to get the Corsair RM550x 2021 (or similar) when using a NAS with more than 2 disks or a discrete GPU.
I personally don't need more than 10TB space, so 2 10TB Seagate EXOS in ZFS mirror would just work fine with 120W as long as you don't run Prime95 the whole time. You might go up to Pico PSU 150 then.
I personally don't need more than 10TB space, so 2 10TB Seagate EXOS in ZFS mirror would just work fine with 120W as long as you don't run Prime95 the whole time. You might go up to Pico PSU 150 then.