If I may.
How about layering multiple tracks.
Develop a base beat and rhythm. Add tracks that enter and shift time while retaining the core beat.
You can lay virtually any lyrics on this.
Basically what I’m suggesting is that you give your songs a deeper and wider back end.
As an example;
King Crimson - Discipline
https://www.youtube.com/watch?v=_-dZNzXylVE
Very interesting idea - I’ve used polyrhythms before, but mostly on the guitars. GarageBand isn’t so hot on bass voices, but layering them could beef up the low end. The only real problem is that I only have 8 tracks before I have to start ping-ponging.
Thanks - that’s something to think about.