YES! I remember now. I have a notes file where all this is written down somewhere. A stock tape recorder can usually only read two channels at a time (stereo L and R), but then you’d still have the “flip it over” side A plus side B runtime.

Thanks for the link, definitely saving that one

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