If you want x86-64 support in fanless, take a look at Celeron (low powered) based industrial PCs. Qotom comes to mind. You can get a passively cooled machine. Most come with a NVMe and 2.5 slot for storage, do doing raid 1 is possible without external storage. I've bern running my J1900 based one for nearly 5 years and haven't had an issue with hardware at all.

Is there some low-power hardware that takes ECC RAM? I want something to replace my Atom mini-ITX board, but I also want ECC.

Do you have a NAS? It can be a good way to get decent functionality without extra hardware, especially if you're doing proof of concept or temporary stuff.

My self-hosting Docker setup is split between 12 permanent stacks on a Synology DS920+ NAS (with upgraded RAM) and 4 on a Raspberry Pi 4B, using Portainer and its agent on the Pi to manage them. The NAS is also using Synology's Drive (like Dropbox or GDrive) and Photos (like Google Photos).

I've had the NAS running servers for Valheim and VRising in the past, but they require that fewer containers be running, as game servers running on Linux usually have no optimisation and/or are emulating Windows.

If I decide to host a game server again, I'll probably look at a NUC. I've done the DIY mini-ITX route in the past (for an XBMC-based media centre with HDMI output) and it was great, so that's another option.

It’s up to you how much you want to pay vs. how much time you are willing to sink into it. A synology is overpriced and underpowered, but you get a nice plug and play solution eith sane defaults. I went with that, fully knowing that price-wise, it’s not optimal. But I don’t enjoy tinkering as much as I used to.