Is it possible to reformat or revert the reComputer J2021 back to a factory state, as it would have been upon original purchase?
Each manufacturer of a third party carrier board will provide their own software. Mostly this would be an “near” exact copy of the Jetson dev kit flash software, although most of the time there is some sort of edit to the device tree (if the carrier board is an exact layout match to the dev kit, then the manufacturer will simply direct you to the dev kit flash software). You will have to check with the manufacturer of that particular carrier board to get the flash software. Expect to install that on an Ubuntu 18.04 host PC if it is based on JetPack 4.x or earlier; or on Ubuntu 20.04 if it is for JetPack 5.x or newer (you could use 18.04 though for JP 5.x, or Ubuntu 16.04 for JP 4.x).