Then you already have GarageBand, it’s stupidly powerful and simple to use you just need an interface. Scarlett would be my first choice, pretty much plug and play. It’s a simplified version of the ever popular Logic X. I own all 4 of Apple’s app, Final Cut Pro/ Logic, iMovie/ GarageBand and use all of them. As the matter of fact, 2 of the covers I posted yesterday were finished on iMovie on my iPad Pro
It’s still so powerful and very easy to use.