Electron Epub compression tool. Open the app, select your compression level, drag some epub files into the window, and Bob's your uncle. App will not overwrite or delete anything, and will temp folders behind that you should probably delete.

git clone https://github.com/L-K-M/ShrinkPub

cd ShrinkPub

npm install

npm start

Drag your .epub files into the application window. The original .epub file will not be changed, a new shrunk .epub file will be created.