It doesn’t look like you need a lot of compute. A DB, being able to run 2 or 3 docker containers and a postgres db and being able to backup stuff and also serve as a NAS.
I’d bet your needs would be covered by the RockPro64 if you aim for lowest possible power consumption. With the official case, you can get 2x 3.5" HDDs and 2x 2.5" SSDs. You might want to do your own PSU, but using a y-splitter with the provided sata cables may be ok if you get lower powered drives.
The only thing is that aarch64 is not really for most people yet, it’s hard to find os support and most programs. FreeBSD runs great on it, so does armbian and you could get gentoo, nixos or other distros to run on it fairly easy.
But instead I’d suggest you go with the odroid h3 (non-plus) and getting the type 1 or 5 case (depending if you want the m.2 ethernet card or not, I’d go for type 1). You only get 2x 3.5" drives, but you can’t get more compact than this (other than the rockpro64 official case). There’s the type 3 case if you want to lower the power consumption and increase the performance (2x 2.5" ssds only), but flash prices are more expensive. You can still get 8tb qlc drives and still have enough for your needs.
Idle power on the rockpro64 with both 2.5 and 3.5 drives populated is negligible, I doubt you’d see much power consumption on the h3 either at idle. At load, the h3 might use more power, but you also get quite a lot more performance. I think the performance / watt goes to the h3, but the lowest possible power goes to the rockpro64.
I’d avoid using the laptop as the brains with a DAS. Just use the laptop as a diskless hypervisor (or limited disk hypervisor) if you really want to put it to use, but I’d suggest you stick to a proper NAS and stay away from DAS.
Also, from the post on your website, I don’t see why you’d need raid-z. A raid mirror is all you need AFAICT and you get better performance on reads. Just have another backup if needed (maybe an off-site). I find the odroid hc4 to be a good low power (and cheap) backup NAS. But the only OS that I found reliably runs on it, with zfs support, is NixOS.
If you plan on getting both, to also have a backup server, then the total amount for the hardware alone would be around ~$280 (excluding the drives, of course). 8TB ssds are around $320 ea., while hdds are around $200 ea. You can in theory go as low as $1080 (for 2 completely new NASes), but you could buy a set of new drives and a pair of used ones, reducing your costs. Or you could go a bit overboard and go for around $1280 for a ssd main nas and a hdd backup nas.
And to save on power, you could power on the hdd nas on demand 5 minutes before the backup needs to run (note that I didn’t try WoL on the HC4, but the board doesn’t have a power button, so all you’d have to do is plug it to a zigbee socket and tell it to give power to the board until the backup is complete, then you can shut it down and after you made sure it’s off, using some scripts, tell plug to kill the power).
I’d consider less than 2 years. I’ve had drives with 5 years uptime that would still be going strong, but it depends on your luck (out of out of 14x 2TB drives, 1 went bad as soon as I started using it, while 2 of them went bad in a few months after - I had a raid-z2 11 drive pool, of which 1 was a hotspare, I had to kick it in, in about a week since I put the pool online, while the two other drives, 1 was in this same pool and another in a different, stripped mirror pool). The reason I said “would” is because I stopped using them (because of OS problems that I couldn’t really solve remotely, so I gave up and powered off the servers - besides, they were using too much power anyway).
I like the idea, but OP said on his website that his requirements are the smallest size possible. A full desktop (even a microATX) would be a lot. Besides, he’s also interested in low power consumption (you could put an Asrock j3455m or one of the newer models in it, but still).