It seems that there is an easy way to expand Xbox Series X|S storage with PCIe 4.0 SSD

An enterprising module developer found a way to insert a standard NVMe SSD into the Xbox Series X|S storage expansion slot—not only increased the storage capacity of the console, but it was also possible to turn it on to use the NVMe SSD. Further unofficial extensions, such as those found in modern gaming PCs.

Xbox Series X|S comes with a 1TB built-in drive as standard. This may sound like a lot, but as we all know, gaming is becoming a truly powerful storage pig. Therefore, in order to cope with the ever-increasing file size, the latest Xbox provides expandable storage by using officially certified Seagate expansion drives.