Program: FFMPEG

Format: .flac w/ FLAC audio (or .mka w/ Opus audio if you really need a lossy codec)

Also, the amount of storage you’re going to save (or lose) by converting from one codec to another (ex. MP3 to Opus, like in the post) depends on the input and output files’ bitrates. But, the quality of the output file will always be worse than the input file if the output file uses a lossy codec.

I’d be surprised if there wasn’t an FFMPEG command to run in PowerShell to automatically convert all of them.

In Linux I always use sound converter. Works great.