UESP says that completing the Thieves Guild MQ before Under New Management can make Brynjolf not talk to you and start the ceremony, but the general Thieves Guild page says: If too many miscellaneous quests for Vex or Delvin have been completed before you become a Nightingale, Delvin may only give out one guild improvement quest. Once this quest has been completed, no matter how many quests you do you will not get another improvement quest.
UESP says that completing the Thieves Guild MQ before Under New Management can make Brynjolf not talk to you and start the ceremony, but the general Thieves Guild page says: If too many miscellaneous quests for Vex or Delvin have been completed before you become a Nightingale, Delvin may only give out one guild improvement quest. Once this quest has been completed, no matter how many quests you do you will not get another improvement quest.