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ffmpeg -f concat -i mylist.txt -c copy output.mp3

It's not quite so simple; I need to insert smaller audio files into a larger file at specific time codes.

Splice you parent file at the time stamps, interleave the samples, then concat.

I'm trying to avoid reencoding multiple times, and saving WAV to disk puts serious pressure on storage constraints

You could use FLAC as an intermediate lossless codec which should reduce storage costs by about 1/2 to 2/3 compared to WAV.