in your project settings, you should be using the mobile compression methods in "compress". Also, try using spriteheets instead of single images. Lastly, try using godot themes instead creating custom controls "scenes". everything is there to make anything you want.
Try tweaking the audio and graphic importer to compress more.
Lastly, the audio importer has options for audio that uses less space but more cpu, OR the other mode uses less cpu but more space.