too many assets is a known issue, see also
as workaround you can try the code server http://code.appinventor.mit.edu/ which does not have these Google limitations
another workaround is to put your assets into a zip file, add only the zip file into the assets and unzip the file in the ASD on first run of the app... this extension can help: App Inventor Extensions: Zip/Unzip | Pura Vida Apps
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by
Taifun.