Note also that the linked article stops at QAM. This is a previously solved problem for phoneline communication; in the 1980s modems went from 9.6kbps to 33.6kbps[1] after adopting trellis coding. It's not impossible that a similar technique would work for audio cassettes.

1: https://www.itu.int/rec/T-REC-V.34-199802-I/en