Right then, keep it simple...

Record from the line out or headphone socket of a cassette player
directly into the line in inputs of a portable digital recorder,
like the very good and affordable Zooms...

You then have WAV file recordings of at least 'CD quality' 16/44,
which can be transferred to computer for conversion to smaller lossless flac files,
or lossy mp3.

It's 2019, there are no serious reasons why folks involved in music
should not already own at least one portable digital recorder.
They are now basic essential equipment.