Becoming Thieves Guild Guildmaster, am I bugged?

I've completed the Thieves Guild main questline (Darkness Returns) and chosen my special power (Shadowcloak of Nocturnal). It says completed in my quest log.

I also completed the extra jobs in each city you can give Thieves guild influence to (Markarth, Whiterun, Windhelm, and Solitude) and done their special jobs from Delvin. They show completed in my quest log. I've even done at least one of each kind of job from Vex and Delvin (but not in each city).

I got 4 vendors in the Ragged Flagon and have done the quest for Tonilia (delivering the Moon Sugar to a caravan) and Vekel (getting the 4 books). I've also upgrade one of my pieces of Thief Armor.

I talk to Brynjolf to become guild master and I get no dialogue options but he just says he's busy and we'll talk another time. There is a guild chest in front of his desk but it's locked and I don't have the key.

What am I missing? Is there some bug I've encountered?

It seems indeed that there is a bug related to thieves guild leadership:

If the player completes the main story arc before doing the side and improvement quests for Vex and Delvin, Brynjolf may not interact with the player and instead repeat that he is busy and that he will talk to you later. Consequently, becoming the Guildmaster will be rendered impossible. This may be a part of a bug where Karliah disappears and does not appear in either the Nightingale Hall or the Twilight Sepulcher. She does, however, return to the Snow Veil Sanctum where you first encounter her.

Possible Fix:

Run outside when finishing your first interaction with Brynjolf. You need to run outside as quick as possible by going to the ladder and exiting. You should see the quest update pop up reading: "Started Under New Management". Afterward, go back inside and proceed to Brynjolf. To talk to Brynjolf, let him go to the middle of the Cistern first, and wait. He then should start talking to you, finishing the quest. If this does not resolve the issue, it may be worth it to try attacking them, paying the reparations, and trying again.

Another possible fix is to get the quest about fixing debt from the guy at the Riften stables. The player will be told to talk to Sapphire. Instead, Sapphire should be sitting on the bridge just inside Riften and when approached will say "I don't have any business with you." or "We'll talk after the ceremony." Attack her 3 times, and this should cause her to start attacking the player notifying them that "Under New Management" has been completed and that the player should talk to Brynjolf. Upon entering The Ragged Flagon, the player will be required to pay 1000 gold to Vex as reparations for killing a fellow Thieves Guild member. After doing so, speak with Brynjolf and the questline SHOULD be completed.

As a last resort, you can use the console to fix this issue:

setStage TGLeadership 0

Brynjolf had kept on telling me that he was too busy to speak to me even after I finished the 4 special quests. I finished out the quest for the barkeep and I finished the job for the fence at the guild and then when I went back to talk to Vex she told me that Brynjolf was looking for me. Don't know if its related at all but it didn't happen instantly after finishing up the specials.

Do you see all 4 new vendors in guild across from the bar? Maybe one of the special quests is hung up,despite what it says.

Did you get all the special items for Del? You have to get of them first before you can become the guild leader.

Since you consoled your way just a note to avoid it altogether IMO.

Delivering Arondil's journal to Vekel is a HUGE bug. Even after the latest patch, the journals do not leave your inventory, and you get nothing in return.

I have been affected by your bug, been bitten, and stay away from it in subsequent playthroughs. I always do that quest last AFTER becoming Guild Master. I do talk to Vekel and get it in my quest log though. YMMV