Just an idea...I realize some would say noooo I did it the old way it should stay that way, but no flames pls.
How about your bottleneck bosses get reworked with no real respawn timers. What?! I mean make them ring/ type quests where you kill a certain mob to spawn next set and so on until your finally at emp / sleeper keying mob etc. Respawn on initial mob fast but the time it takes to complete total cycle would be anywhere from 1-2 hours that way those who need it can do it in turn.
Fennin is already this way and should NOT be changed to lower spawn times but to instanced because...you would have to increase the difficulty of the path to fennin else box teams would just farm it constantly until they got everything they wanted from it AND if you increased that difficulty it might be too difficult for people doing the event for the first time.
Just a thought
Taladome