Using the strongest signal (instead of average) when mixing stereo to mono

I’m digitizing old cassette tapes that were made using a mono “shoebox” tape recorder, and using an old walkman with a stereo output to play them into the computer (and then mix them to a mono track).

However, one tape has an issue where the signal is randomly stronger on the left or right channel (it drifts back and forth between them, so I’m assuming it’s a problem with the physical media). This causes a problem when mixing to mono, because that averages the two the signals. Because one side is “normal” volume, and the other side is very low, the average results in a much lower volume than it could be.

Other than manually choosing the strongest side, is there a way to mix down to mono where it chooses the side with the strongest signal, instead of averaging them together? Or another technique to approach this?

I can also use ffmpeg or sox if those provide a better command line way to approach this.

Split stereo to mono, then mix and render the two mono tracks . low volume avoided.