The first book:
- First night at Greenrest will be a breeze instead of a frantic meatgrinder, you can slaughter kobolds and cultists with ease. The champion and dragon are supposed to be almost cutscene battles but can be wiped handily by L20s. The party could even split up for the branching events to deal with basically everything.
- infiltrating the camp could end the book there and then, between all the stealth and mind control options available you can figure out whats going on and kill off whoever survived your presence in town during the attack
- assuming the party infiltrated 'properly' and rescued the injured guy, getting around is easy with things like phantasmal steeds, flight and teleportation. Theres no reason you couldnt catch up to the cultists fleeing with the loot and again wipe them out unless you for some reason havent figured their destination (like by accidentally killing the priest in the cave youre not supposed to go into during the infiltration, and cant just speal with dead on them or something for the info anyway) and decide to tail them.

Book 1 sounds like a oneshot session in length. Havent done book two myself sorry, but at least that potentially ends in fighting tiamat which will last more than one round.