DTMF was a terrible concept for recording anything - it was intended as a way of carrying single keys down an audio line that could be decoded through a bandpass filter, and would be incredibly slow - probably would struggle for 75 bps.
Most tape recording systems use FM and this could have been improved with MFM, and most typically operated around 1 kbps - not terribly fast, but in 5 to 6 minutes, would carry a full memory load of data.
Some variations existed - eg, Spectrum used one flux gap for 0 and twice that length for 1, so different data profiles would have different lengths. But these were easy for a CPU to decode in software in real time.
The 8 track concept had better luck however - a 2-track version was made by Sinclair, called the Microdrive, which had a data rate of closer to 100 kbps, and a formatted capacity of around 80 kbytes, though I've managed to fit nearly 640K into the data format through quirks in the formatting.. That is it's absolute theoretical limit though, and would take nearly a minute to load in a full 640K, or files that required full traverse of the data structure.
These were very successful, and the cartridges were a fraction of the size of an 8 track cartridge, but used the same principle and had two interleaved tracks that were decoded in hardware and used biphase ( type of FM ) encoding. In theory, they could have held nearly 256kb of data with read times as short as 8 seconds, which would have made them quite competative with floppies for early PCs. They were often called Stringy Floppies, and in fact, that was also another type/brand of 8-track style cartridge - as was the Rotronics Wafadrive. They compared well to floppy disks of the era, but you could fit an entire drive into your pocket. I have some here, and will connect them to my PC design when I get it into hardware
