Dumpster Santa delivered a nice Coolermaster Mini-ITX with Gigabyte GS-H97N motherboard and an Intel Core i5 inside.
Since it already has 2 Ethernet ports, you could use that PC as a pfSense router.
The CPU should be able to handle the throughput of your internet connection. It also has good OpenVPN functionality.
https://www.pfsense.org/
I'm fairly sure you have the PSU in upside down. The fan should draw air from the CPU area and push it out of the back.
See the image on here:
https://www.extremetech.com/computing/128313-extremetechs-guide-to-air-cooling-your-pcThe old ATX case design requirements I've seen show this too. This is back when the PSU fan was the only case exhaust fan.
Found in several EVGA power supply manuals:
NOTE: It is recommended to install the power supply with the fan
facing down. However, if your case places the power supply at the bottom of
the case and there are no ventilation holes available, it may be best to install
the power supply with the fan facing up for greater efficiency and reliability.
Here's an old build out I did. The fan on the PSU is there and is part of the ATX specification. The extra fan is not part of the specification. It can be removed and the case will still be an "ATX case". The CPU is sitting next to the PSU fan for a reason. It would be the only way a passive heatsink would get sufficient airflow. They could have moved the CPU further away from the PSU, but they put it there for a reason.
This machine ran overclocked at 133%. This arrangement kept the CPU cool enough. Well under 100 C.