If the per player difficulty settings are able to make it into the initial release, I believe we will have a roundabout way to adjust the pacing.
If the player sets their own difficulty to an easier difficulty level, they will get numerous bonuses to their development early on as well as a plethora of habitable worlds nearby. If they set themselves to normal or above, they will have an average start. On hardcore, they will have an even slower start.
Meanwhile the AI can be controlled completely separately. They can develop quickly (on higher difficulties) and quicken the pace the player needs to play at, or they can develop slowly (on lower difficulties) allowing the player to play at a more leisurely place.
That is the easiest way to resolve this issue given a system that is already likely (though not guaranteed) to make it into the game at this point. It can always be tweaked once implemented.