I feel bad pointing this out, but nothing you are suggesting will really be very much of a "jumpstart" save getting 16 level 30 leves for your choice of Dark Knight/Astrologian/Mechanist and not completing the Challenge log.

Frontloading the Challenge log will actually decrease the amount of XP you get from completing challenges. As the XP granted by the Challenge Log scales based on level, completing it at level 50 will give you less than completing them at level 51 or 52.

Leve quests grant far worse XP rewards if you over level them. The current highest level leve quests (those at Saint Coinach's Find) are level 49 max.