The cart comes by itself, so you would need some means to write updates to it. You can do it with a DS (that has a gba slot) using homebrew software like you said or get some other cartridge flasher like the one insideGadgets sells / the epilogue operator.
You missed the 2022/12/6 restock! Unfortunately there seems to be a shortage for cartridge parts at the moment. There will be more stock at some point but I don't have a time frame to give you sadly. Make sure to turn on email notifications on insideGadgets by giving them your email in the out of stock page!