The TrueNAS Mini X+ has no PCIe expansion — four SATA bays only

The TrueNAS Mini X+ is a dead end for expansion — no PCIe slots at all, so what you see (four internal SATA bays) is what you get. If you outgrow those bays, you’re looking at an entirely new NAS rather than a simple card addition. Fine for a small home setup where you know your ceiling upfront, but tight for anyone who plans to scale.