Energy Efficient Homelabbers: How do you do big storage? Doesn't need to be super fast.

The longest serving part of my homelab is a 6 disk raidz2 array running on TrueNAS. The disks are all WD Red 3 and 4 TB. Some of the disks are 12 y/o. My original plan was just to buy cost-effective disks and slowly cycle them all up to higher capacities as they need replacing (I know this wastes some space), and now I don't really want to do that anymore. This machine has become my target for energy optimization and I basically use it as an offline backup now. For online storage, I have a media library on a 2.5" USB drive, (backed up on the offline NAS), and this is always on so I can stream from my library anytime.

But I'd like to have something more robust as my energy efficient always-on storage besides that little external HDD. It doesn't need to be super fast, but the energy efficiency of flash storage appeals to me. I've thought of cobbling together a bunch of random sata flash drives with unraid. But I guess it could also just be a mirrored pair of new HDDs with terabytes in the teens, or maybe just some more external USB drives.

One plan I had was to get some used 2tb Micron 1100s and start a flash array. So far I just have one that I got for around $100. Is this more of those a good plan? Also are mirrored pairs the way to go for flash pools or is there some parity-using configuration where it makes sense even with the write amplification? A lot of what goes on the NAS is write-once type data.