For the final submission, consider making the webgl build launch in fullscreen from the itch page settings. See the turn in instructions or finish the project video for more info on that.
Your proposed modifications mostly sound good. The first one though, you should do a little more to make your levels stand out, perhaps more geography changes/level layout changes along with the enemy variety. If you are making new art yourself that will certainly be significant. If you are pulling art from elsewhere be aware that you need to shift the entire aesthetic to go for full points on that as a significant modification.