Welcome to Terry's forums Mike,
Normally you would have a larger breaker in the main box, then limit the smaller current needed for the garage in it own box.
I would feed a 30 to the 20, not a 20 to a 30, as it would never trip if really needed.
I am not sure if I understood your question, forgive me if I HMHUMA.
That is just my thought, but the pros will show up shortly, To help you.
Have a Great Day. Good luck on your project.