Notes for myself but maybe useful for *you* too?

https://github.com/dfaust/soundkonverter

Add your directory or directories, let it scan.

For music select Opus with 48 kbps if that’s enough for your use case.

Create the file list, then fix special characters with https://github.com/dfaust/soundkonverter/issues/96: sed -i 's/file:\/\///; s/%20/ /g; s/%23/#/g' ~/.local/share/soundkonverter/filelist.xml

For audiobooks use Opus with a low bitrate of 16 kbps and the Additional encoder argument -ac 1 for mono.

Copy directory structure will create full path directories in the destination directory, e.g. if you convert /home/foo/music/ to /tmp/, it will create the output in /tmp/home/foo/music/.

For future updates: Select the same directories, fix the xml again, make sure that Settings -> General -> Conversion -> Conflict Handling is set to “Skip File”.