Lore and writing wise? You're completely right, I'd think the characters would be expected to go in Tartarus on a regular basis, progressing from miniboss to miniboss I guess.

Gameplay wise? We're kinda pushed doing it at once very quickly to get some money, and more importantly, to free up some time for social activities. So to optimize a bit the flow, I do the same : go to the end in one day, then go back towards the end of the month for the quests, finding missing people, etc