There's some space occupied by the servo tracks (which align the heads to the tap) in LTO, but if we ignore that...

Current-generation LTO9 has 1035m of 12.65mm wide tape, for 18TB of storage. That's approximately 13.1m², or just under 1.4TB/m².

A 90 minute audio cassette has around 90m of 6.4mm wide tape, or 0.576m². At the same density it could potentially hold 825GB.

DDS (which was data tape in a similar form factor) achieved 160GB in 2009, although there's a lot more tape in one of those cartridges (153m).

Honestly, you'd be better off using the LTO. Because they're single-reel cartridges (the 2nd is inside the drive), they can pack a lot more tape into the same volume.