Itunestify Portable -

: Preparing audio files to meet different platform standards, such as the -16 LUFS requirement for iTunes vs. the -12 LUFS standard for Spotify. Why It Still Matters iTunes - Apple

You must source your own music (CD rips, Bandcamp downloads, old MP3 blogs) and then iTunestify the whole batch so that when you scroll your click wheel, the experience is seamless. itunestify

If you are looking for specific software that fulfills this "iTunes to Spotify" bridge, the following are highly rated: : Preparing audio files to meet different platform

: Moving saved music from Apple Music to Spotify or vice versa. Smart Search itunestify