@grant@clevyr.com
I think it's better to focus on creating a Laravel package, as a composer package won't have everything that Laravel packages needs to have.
You can setup a Laravel package skeleton easily using https://github.com/Jeroen-G/laravel-packager
Next would be to setup similar folders in the package, i.e. app/Models will become vendor/package/src/Models, you will have to modify their namespaces etc
There is also this part about registering your resources https://nova.laravel.com/docs/3.0/resources/#registering-resources
If you are using things like livewire etc, that has to be registered too.
For migration, you have to register the folder $this->loadMigrationsFrom($folder);
As for other composer packages, just add them to your new package's composer.json
Registration is probably the trickiest part if you are new to creating packages.
I think you should be able to handle the rest with no issues.
Hope your page builder will be an open source one. :)