In terms of timing:
Generally speaking, the longer you wait after someone has signed-up on a landing page, the less likely they are to convert when notified that the app is ready. You can "drip" teasers and other content to try and keep in their memory and keep them excited via email and you can also engage them directly via email outreach to do further customer development.

In terms of what a landing page should include, it should be a product shot or shots, and a clear message as to what you are offering with an email sign-up.

I suggest that after they have provided their email, you should try and create some extra engagement with the user. In your case, being that you're inviting musicians to collaborate efficiently, you have an opportunity to create some virality by getting the person signing-up to invite their collaborators to join early as well.

You don't want to launch to the app store until you have a polished app that is fully functional. You want to get people using a test version (via Hockey or Enterprise distribution) as quickly as possible. You can select your most excited sign-up people from your landing page as candidates for testing the app.

Happy to talk about this further in a call