You might check your system board’s manual. Some include a header for serial ports and just need a cable and back panel connector. That on-board serial port might be supported, (not saying it will be…).
Now on the topic of unfortunate and disappointing, TrueNAS was not written for us, the SOHO free users. We are in essence testers for the Enterprise data center crowd.
To be clear for future readers, TrueNAS is appliance software. Not a Linux distro with wide capabilities. Ever since iX started the Linux based SCALE fork, we have gotten lots of people, (not just hundreds, but thousands), who expect SCALE to do more that it’s stated purpose. And I mean even beyond VMs or Apps. That would be fair for a normal Linux distro, but certainly did not exist for the FreeBSD based TrueNAS Core.
In any case, good luck.