I'm thinking of quitting my job and telling my boss why 6 months later
Why would you quit and not tell your boss why until that much later?
I enjoyed working at my job but my boss can be so much of an asshole.
It's natural to want to quit a job if you're no longer interested in being there. Although not providing a decision may upset some people, its still your right to keep that information to yourself.
I rather not tell them why ASAP
And that's okay. Collect yourself, plan your next moves, and then give your reason.