When you boot to safe mode, log in to your account and check if the issue is also occurring in safe mode or not. Safe mode will tell us if the issue is related to a startup item or plugins, and sometimes is all that is necessary to correct some problems. The article above also has steps to check if the issue also occurs in safe mode. I know the article title says for macOS Sierra, but the steps are the same in macOS Mojave. Since it's happening with saved projects as well as new projects, I recommend first trying the troubleshooting in this help article: macOS Sierra: If an app freezes or quits unexpectedly - Apple Support
Thanks for asking the Apple Support Communities for help with GarageBand on your MacBook Air, and issues with the app freezing since upgrading to macOS Mojave.
