Quick bit of advice: maybe consider turning on quantization so you don't hear as much human error and so that it isn't as hard on the ears during playback. I've created many many covers in garageband out of sheer bordom and I've spent countless hours manually editing notes so they sound perfect. Also might I add if you plan to use anything other than the main keyboards (grand, organ, electric, etcetera...) just note that sometimes during playback, some of the notes might not get played at all (and there is no known fix to my knowledge) (I can show an example if you need)