Apple didn’t sell larger devices so that’s what they’ll test and document, but there are a large number of folks that have retrofit larger-capacity SSD storage.
macOS with HFS+ and APFS can address far larger storage volumes than any that are currently available in the market, too.
OWC has a reasonable reputation for supporting what they sell, or you could take on the savings and the commensurate risks and efforts of finding and integrating and testing and supporting your own storage choice; not all devices will work in all configurations, unfortunately.
If the storage was encrypted, then erasing it is not necessary past clobbering the keys. And erasing an SSD is largely futile, as SSDs don’t work the same as hard disks used to; a deleted file is TRIM’d and that erases the contents when the file is deleted. Trying to overwrite zero to avoid remanence just wears the SSD. Rewriting the same sector the file was stored in won’t work, as wear leveling and caching will provide you with a different and unrelated sector to overwrite, which’ll immediately get TRIM’d and cycled back on the cache of erased sectors ready for (quick) reuse.
A failing hard disk—either a hard disk, or the hard disk portion of a Fusion drive—will be replacing sectors that have failed with other spare sectors, which means overwriting those replaced with errors sectors is difficult at best. Those sectors may or may not be writable, and you’ll have to override the drive-level error handling to even get to those replaced sectors.
This is part of why File Vault encryption is so useful. That encrypts the contents of all sectors including those sectors that might eventually fail. Whether SSD or hard disk or Fusion.
