I don't see the soundtrack on Youtube Music or Bandcamp, the places I usually stream/buy music from, and I like the music and want to listen to it outside the game.

Unfortunately there is no dedicated soundtrack album outside of the game, as it is made up of licensed music from various talented composers.

The stage music and final boss music is made by Isao Yamaguchi.
The general boss music is made by Andrew Sitkov.
Some additional tracks are from Tim Beek and WOW Sound.

I tried reaching out to Isao some time ago, if he would be able to provide a soundtrack for personal use (e.g. on his site), since most of the tracks are from him, but unfortunately did not get any response back then.