Just get an old 1L class dell/hp/lenovo, a riser to adapt the proprietary expansion slot to standard PCIe, and an old x520DA2 nic. You can find all of this on eBay. Then, just slap pfSense or OPNSense or whatever you want on it.
Source: running an old Lenovo m920q with that exact setup. 2.5G copper module to the modem, fiber to the SFP+ port on my switch. Works great.
Edit: and the switch is the 9-port version of the one you posted.