I work with old cassettes all the time. This sounds to me like a problem with the tape movement . If the cassette is pre-recorded like the ones in the picture, then the problem is likely during playback rather than when they were recorded, and so it’s either your deck or the way the tape is moving inside the shell.
The audio sounds like the tension on the tape is changing as the reels spin, which causes the tape to move relative to the playback head, and when the treble frequencies are cutting out, the tape is further from the head or slightly askew relative to it. Another possibility is that the playback deck isn’t decoding Dolby properly. I don’t remember seeing anything about that, but if you haven’t tried playback with Dolby disabled, it would be a useful comparison to try that.
When working with old cassette tapes, winding the tape forward and back once before playback is a good practice, and listen to the mechanical noises as it moves. If the tape is rubbing against the shell or something you may hear a squeal or whine. Or, if the tape is sticking somewhere, the motor noise may suddenly change as the tape passes that point. Those are both hints that the tape isn’t neatly wound in the shell.
Cleaning the deck with isopropyl is a good idea, but the heads are probably not causing this. The capstans and pinch rollers are more likely because they allow the tape to slip if they are dirty. Old, caked on tape oxide can take a while to remove. Put some real pressure on the Q-tip until it comes away clean.
This sort of cycling pulsing problem can also be caused or exacerbated by loose belts between the motor and the capstan. It’s worth taking off the cover and having a look and a gentle tug on the belts. Replacing the belts often helps many problems with old cassette decks.
It is very difficult to fix up these problems using software except manually, which is too tedious to be practical. It seems like it should be effective to do something like boost the high end across the tape and then use a dynamic equalizer to tamp down anything above a certain volume threshold, using a tool that responds slowly and progressively as the audio changes. That is not my strong suit. Others may have better ideas.
Hope some of that helps.