So I saw this Dell Optiplex 3080 Micro unit on eBay. Typically, those come with 10th-gen Core processors, but the one I saw was the rock bottom of the line, featuring a modest Celeron G5900T (dual-core, 3.2 GHz) with 8 GB RAM and no storage. It looked like perfect router conversion material; I was sold. 
The built-in NIC was a Realtek 8169. I happened to have an add-on 8169 in my supply closet, along with a 64 GB slim SATA drive. So this is what I ended up with internally:
Externally, I was very pleased with the location and shape of the opening for the add-on NIC:
It’s neatly aligned with the onboard NIC, and the two openings for the screws are actual round holes, as opposed to grooves with rounded ends typical of earlier generations.
Installation of the add-on NIC was uneventful. Installation of OpenWrt, almost uneventful. I wrote 25.12.0 (squashfs) onto a slim SATA drive, installed it into the device, and it booted right away. The only issue was, the console was being flooded with messages saying that the OS doesn’t have control of ASPM and can’t turn it off. Never mind; I turned it off in BIOS.
By default, the add-on NIC was designated LAN, the onboard NIC, WAN. I reversed those designations on an off chance the onboard NIC has some functionality more useful in the LAN context.
Just for kicks, I tested the device with Wireguard and posted the results into the Wireguard performance database thread. The device clocked in at a respectable (for a unit with Gigabit networking) 2.84 Gbps.
All in all, I think this could be a solid choice for a hobbyist seeking an inexpensive workhorse device for Gigabit VPN workloads. I am guessing, Gigabit SQM, even if still single-threaded, shouldn’t be a problem, either…