I’ve only been running a few services like AdGuard Home so I can’t speak to very heavy loads. I’ve had no issues with it. I probably wouldn’t run my Plex server on it but any regular network services should be fine.

I have mine sitting on a wire metal shelf so it gets plenty of air flow. I did misconfigure the Unbound DNS server to recursively query itself and CPU load was at 100% for several hours. I noticed it was warmer physically but it was not hot enough to give me any concerns. Still well within what’s safe to run.

I personally use a Qotom PC. Not sure if it fits your exact requirements since you can’t get a PCI card into it, but it does have 4x1Gb ports. Works really well with opnsense.