Well, I have an update.
The box is built and I'm very happy with it.
Pentium Gold G5400
Kingston 8GB 2666MHz ECC DDR4
Samsung PM961 128GB SSD
Intel NC365T Quad-port 1Gb
The case doesn't actually have a PCIe slot (shame on you, InWin!), but quick work with a Dremel (to cut the slot), a couple of nylon standoffs (to support the card vertically) and M3 screws/nuts (to hold the standoffs in place) solved the issue. I ended up with the Supermicro board because on a normal mini-ITX board, the audio block is too tall for a 1U and I was in no mood to start hacking away at it.
The box is running Proxmox VE and is dedicated to networking functions; so far I have two instances of pfSense running (work and personal) and I plan on playing with a Linux instance running ZeroTier...I might host a virtualised RouterOS instance also. Supporting the box is a Dell Powerconnect 2824 (for which I replaced the fan with a 5V Noctua 40x40x20) with a few VLANs configured (a couple for work, the rest personal).
There's a few last finishing touches to be made to the box physically (more Dremel work) but overall I'm really happy with it.