I found a fix for this on the nexusmods forum.
Skyrim SE with the 1.4 patch. After completing Darkness Returns and all of the 'small jobs', the 4 'special jobs' for Delvin, and the Moon Sugar task from Tonilia, Brynjolf STILL won't start the quest. You can ask him "How's the Guild doing?" but that's it. To fix this you need to get Karlia and Sapphire into the Cistern.
Run these commands:
And then run:
setstage TGLeadership 0
This will start the quest.
Thank you! for the tip! Worked like a charm. Almost two years later (post) and this still happens. Done and Done.... Thank you