This is my thing. I have about 122TB of spinning metal (with the same as an offsite backup) with SSDs as ZIL and L2ARC. And it's awesome. HDDs I think will genuinely be important for for the foreseeable future.
There is a lot of power to waste for the savings you made, when not buying expensive SSDs (20€ a year is not much). Where we use HDDs, we don't care about noise. Durability? We use huge RAID systems with lots of redundancy.
I personally like to swap new drives after 5 years to avoid failures. So when you find a 16 TB SSD for 350€, you send me a message.
My 4 bay HDD NAS uses around 45W, 50W with some light load, 70W spinning up. That's about 1kWh per day, or 150 EUR per year.
I use it in my room, so I very much care about noise.
More durability = less redundancy (less cost) + less frequent swaps (less cost). My anecdotal evidence is 1 failed SSD in 15 years (160GB Intel, basically first Gen). Every other SSD is still working. I have a drawer full of failed HDDs.
My HDDs run 24/7 without spin up btw. I'm just talking about the costs. My drives don't fail that much as yours. The recent drives that failed were WD Blue that were very old and only used for backups. And yes, all backups were still readable, even the drive was reported as failed. Compare it to SSDs that often fail "spectacularly".
Don't let your dreams be dreams, I didn't know Jack shit about nas and just built my own with an old pc, I tried truenas but ended up paying for unraid, it was just easier for my needs.
Yes. SSDs are still excellent for small form factor and speed, but for long term reliable storage in massive volumes, old fasion hard drives are only second to tape storage.
Source: I am in charge of four 1.2PB storage clusters, each consisting of 144 10TB Toshiba drives. They're due for an upgrade.