The bluetooth to FM transmitter works well for you? I've tried them several times over the decades, even the expensive ones seem to suck. Maybe not as much as your bluetooth to cassette, I've never seen one of those for sale or used one.

Sounds like an issue with your cassette deck. You should definitely be getting better audio quality with a cassette adapter, mine sounds better than a normal cassette tape. Every radio frequency transmitter I have ever tried has had severe artifacting on the high end (treble), especially prevalent on "S" sounds; they come out really static-y. At any rate, your better off doing literally anything else than repairing your cassette deck if it's cooked, but its worth a go to try a standard aux cord cassette since they're under $10.

I've actually opted to record my playlists onto cassette tapes, and I wound up using these more than the aux adapter.