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.