I would say, wait until you've fought and defeated Terminus and it's guardians at least once before moving to Hard. There's already a difficulty spike in the floors after the Warden, and Hard mode might be harder even than that right from the start. And then the same difficulty spike in the later floors exists in that, except worse.
Hard mode is meant for those that have mastered Normal, and it sort of assumes that you're going into it with alot of experience with the game. It is likely to continue to get a bit harder, as I'm probably going to add some more stuff to it.
That all being said, Crystal Mother and Mirror (particularly Mirror) are a couple of the easiest bosses. They make for a good warm-up for later stuff.