If you really want to save some space, you can save a lot by opting to use command-line programs instead of the GUIs that basically just act as front-ends for the command-line programs. E.g. most FOSS audio converters just use ffmpeg behind the scenes, so really they're a lot of extra libraries and such just to give you a graphical means of building up the CLI command. Same goes for videos. Same for images, though a lot of those might use imagemagick instead (even though ffmpeg also does images).

That being said, I use VLC for converting videos and audio files. I honestly can't tell the difference between uncompressed (e.g. .wav), lossless (e.g. .flac or .ape), and high-bitrate lossy like mp3 v0 or mp3 320. Heck, I can't even tell the difference between the aforementioned and mid-bitrate lossy audio like mp3 192 or Vorbis (the .ogg format) 192.

Quality is of course dependent on how sensitive the listener's ears are, and how quality the equipment you're listening with is. You'll want to run some test conversions youraelf before committing to any specific format.

If your files are already in lossy formats, then you really don't have much choice for compression, outside of bundling them all together in a tarball and compressing them with something like gz or zstd. This is only good for storing them, if you try to listen to them, your system will have to extract them, and that either takes time before it plays, or causes stuttering while it plays.