The /128 is what your router gets. After that your router would request a delegated prefix to serve to the LAN, which would be a /64 or bigger. Casual internet search says Spectrum supports delegating /56's.

Even when I request a larger prefix, Spectrum gives me a /128

A delegated prefix cannot be smaller than a /64 so no, that's not possible.

If you only see one /128 as the IP of your router's WAN interface and nothing else, then that is the /128 assigned to your router by DHCP and nothing to do with prefix delegation.

Yes, a /64 comes in. But the first IPV6 address I use in a prefix, that is the only one that they ever seem to accept traffic for. So functionally, it's a /128.