Tuning 808s in Garageband is really quite easy, and it doesn't take as much work as you think.
While creating sliding 808s in Garageband is possible with the Deep Sub Bass Synth, it's much better to use third-party plug-ins.
You can use parallel compression in Garageband in two different ways.
How to add sustain to notes in Garageband, Logic, and other DAWs, using the software, Melodyne.
EQing a piano doesn't have to be a big deal. There are just a few things to keep in mind.
A tutorial on how to automate the equalizer in Garageband. This article also explores the automation presets.