Depending on your needs, you could check out my homelab build.
If you think an Intel Atom motherboard would suit your needs, you could build something like I did - a 1U Supermicro rack case, with a Mini-ITX motherboard - I currently use this one: A2SDi-4C-HLN4F.
They available with up to 16 cores. There is also a newer generation coming soon - with 4 or 8 cores, and with or without SFP ports: A3SPI-4C-HLN4F.
This is about cutting it as close to the limit as you can, if you’re really serious about low energy consumption. The A2SDi-4C-HLN4F quad-core board uses ~15W, and my entire server uses around 15-20W with 4 SSD disks. Just bumping to the 8-core board increases power consumption by 10W, and using the next-gen A3SPI-4C-HLN4F board bumps usage another 10W to ~35W just for the mainboard.
In my opinion, a Mini-ITX Intel Atom server is a good compromise between a more powerful server, and something compareable to Raspberry Pi. Power-wise, it should be comparable to Odroid H3, which was also mentioned.
I’m running a bunch of Docker containers (including Grafana Dashboards), and since I switched to ECC memory, everything has been running smoothly with no issues at all. Also even with the small Noctua fans, everything is very silent.