How demanding? Hard to say as I don't really play any other role but tank, but I feel it's pretty chill.

If you want to practice, just join a fresh prog party for whatever the current tier's first fight is. In prog, rotational mistakes are a given. In fact, some would argue that your priority should lie in learning the mechanics, first and foremost. Those same people would argue you could be spamming your projectile for all you care, assuming there's no DPS check built into the phase. The important thing is to learn the fight. But if you ask me, it's best to just get your rotation down from the start as you learn, so when you do reach the stage where you're ready to clear, your DPS is up to par. In prog, dps does not matter. In clear parties, dps does matter.