Re: a rocket that already exists.
" BFR may well be ready before before the SLS."
More true than not, when you compare government projects to private industry (in general).
Back in the 80's, when I was in the Navy, the San Diego submarine base had an ongoing project to construct barracks for single officers. They were living at the barracks at another base a few miles away, or in apartments in the city. So this had been under construction for the entire time (>3 years) when I was at the San Diego base.
In 1985, McDonalds got permission to construct one of their restaurants on base. It was up and running in about 3 months, from laying the foundation to flipping burgers. And, by comparison, the officer's barracks WAS NOT EVEN REMOTELY COMPLETED after 3 FORNICATING YEARS!
That kinda says it all, I think. Musk wants to make money, so HIS "BFR" rockets will be designed, tested, flown, and approved LONG before NASA gets even REMOTELY close to having SLS ready.
(without the 'urgency' of the space race, like it was in the 1960's, gummint projects will milk the funding for all it can, keeping people employed indefinitely regardless of how important or useful the jobs are, creating circular bureaucracies that justify one another's existence, and LOTS of spinning wheels without motion)