Fix a post-recording of an audio cassette

Running Audacity 3.0.4
On Windows 10

I recorded some audio from a Polish church music cassette with some acapella songs and some accompanied with a guitar or organ. One song even has a piano at the beginning and end. For many songs, especially on the left audio channel, the volume oscillates every 3 seconds near the beginning and every 1.5 seconds near the end. It quickly lowers in volume by about 24 db. I have heard that using compression would help solve these problems, but of course, I would like input on the matter, since I’m still not very good with using compression.

The full cassette:
Example near the beginning of Side B with slow volume oscillations:
Example at the end of Side B with quick volume oscillations:

0:00 of the full cassette is Side A, 31:15 is Side B

I will say that halfway through Side A (about 17:48 into the full cassette), the audio is fine, so no editing is required there, but Side B and the left audio channels are the ones where the problem is most glaring.

I did apply the Low Rolloff for Speech EQ setting to minimize cassette rumble.

I abandoned the left channel, compressed the right, then duplicated it, then pseudo-stereo …

If you have the time, you could use envelope tool to make fine adjustments to the volume,
( as the compressor does not “know” when the musical performance is supposed to be louder/quieter ).

This sounds amazing! What is your exact process to do all this? What settings did you use?