If PCI-E is a must your best bet (with least headaches) is still something x86 based, and for the lowest power you'd want something like a celeron-N or J series. For example this board: https://www.asrock.com/mb/Intel/J5040-ITX/ has sata, m2, pci-e, non-soldered RAM and will idle at ~6-7W (excluding storage). If I were doing it, though, I'd spend a few extra watts on an i3-10300 or later, which will idle at maybe 10-12W (again excluding storage) but give you much better performance and much better hardware assistance for video transcoding if you're going to be running plex/jellyfin or an OTA DVR. I use an Asus PN-41 miniPC with an i3 10300 with a couple of USB-C HDDs as an offsite backup server, and it idles right in that range plus about 5W per attached disk (when not spun down).