For the remaining duration of your lease- could you put some kind of sound-deadening material (I'm thinking Homasote or Celotex) on the walls behind your KBs, where they adjoin the neighboring units? Homasote is grey, but you could make it less noticeable by painting it the same color as the existing walls. You could just lean it up there. But it would be safer if you attached it to the studs with a couple of long screws. When you move out, there would only be a couple of small holes to patch. No different than if you had put up a bookcase.

I lived in an apt. for a long time and I feel your pain. It's a shame to have to put your hobbies on hold, or limit when you can practice them, especially while this pandemic is raging outside. Obvioulsy you could also play through small monitors, or headphones. My $.02.