@Church10129 It could be handled like how Warframe has their Endless Missions (Defense, Survival, Excavation, and Interception if I recall) where every five waves/five minutes players can pick up their rewards and get the option to stay or leave.
I feel that for the case of Phantasy Star Online 2 it is very late for this but I would see it as potentially a smoother run than the current Mining Base quests considering that Endless Quests are similar in structure and gameplay to Warframe's Sanctuary Onslaught (though in Warframe it is seamlessly teleporting from cell to cell with randomized enemies for pure action where in Phantasy Star Online 2 you don't exactly get very good rewards from this and instead serves as a test of the player's knowledge and skill in its hardest content).