I’m not exactly sure what sustainability and efficiency means, but the best thing you can probably do for sustainability is be sure you have applied the latest BIOS to address the Raptor Lake early CPU degradation bug.
The differences between your ASUS ProArt Z790-Creator and the full build Supermicro X11SPH is more than just ECC RAM. The Supermicro board has other Server motherboard features like IPMI, support for up to 2TB of RAM, dual 10G NICs, more PCIe lanes and many others. The main advantage for 45HL is that it has an onboard HBA for 8 SAS/SATA drives in addition to the regular 8 SATA ports, so they don’t have to include a separate HBA in the build, freeing up a PCIe slot and reducing costs vs a discrete HBA.
The main purpose of ECC is to mitigate against data corruption in memory. How big an issue this is for ZFS is debated across the internet. ECC RAM certainly can’t hurt, and is the most recommended. But memory data corruption definitely is a bit of an edge case. Many people successfully run ZFS with non-ECC RAM with no problems. The trouble is, if the RAM data corruption isn’t caught, the system can start writing corrupt data to the disk that is then not caught by a scrub and gradually corrupts your entire pool. It really depends on how critical your data is to you.
Do you really have no other specs for what the HL15 system needs to do? That system seems like overkill for just transcoding.
Chipset or socket? LGA1700 (of your CPU) is the most recent intel desktop socket. If you need CPU transcoding you need to stay with Intel, although in many other ways AMD produces more what I would consider sustainable and efficient CPUs, but my definition–for how long they support the socket and price/performance–may be different than yours. AMD chips don’t have Quicksync though.
You could step back to an “H” or “B” chipset, but you’d lose overclocking of your K chip, and since almost any other motherboard you buy wouldn’t have an onboard HBA, you probably want a board that supports x8/x8 bifurcation so that along with the HBA (which will take one x8) you still have another x8 slot free.
There are more recent Intel server sockets than the LGA3647 on the X11SPH. There is an LGA2066 and LGA4189, but I don’t think Supermicro has made an X12, X13 or X14 board similar to the X11SPH for those sockets.
I guess if someone said “transcoding NAS,” I’d probably think something like 12th gen Intel and an “H” chipset. If someone said “sustainable and efficient transcoding NAS” I might think more along the lines of an AM5 system with a discrete GPU. Neither of those give you the features of a server motherboard though. I think many people are getting the full build with a CPU upgrade to the Xeon 4210 or 4216 and adding in a GPU if they need transcoding. It doesn’t have to be a high end GPU.
Not sure if any of that helps.