> I see now there are only two internal drive bays, so that would mean tossing the 256GB that comes with it if you wanted to max out the internal storage.
Or using some of the several PCIe slots to add lots more SSDs. It looks like the default configuration has enough spare PCIe slots and lanes to accommodate 8 M.2 SSDs in suitable risers. That can get you another 16TB for ~$1.8k (consumer-grade SSDs).
You can't buy drives separately. Storage is almost certainly raw flash, exposed as NVMe through the Apple T2 controller.
EDIT: I see now there are only two internal drive bays, so that would mean tossing the 256GB that comes with it if you wanted to max out the internal storage.