I think the Pact Magic feature would basically be solved by giving the Warlock one or two extra spell slots, and then just giving them a recharge mechanic that is under their control instead of the DM's.
Say, for instance, you start with 2 slots, and get an extra at 5th, 9th, 13th, and 17th level.
You can take one minute to reattune yourself with your patron to recover all your spell slots (edit: specifically Pact Magic slots, obviously) You can do this once per day at 1st level, and gain an extra use at 10th level and 15th level.
Then bring back the old Mystic Arcanum, and keep the rest of the new changes, and boom. Perfect Warlock, IMO.