I just finished the Under New Management quest and everybody I talk to just says Brynjolf is looking for you for the guild master ceremony and when I talk to Brynjolf he just says "Sorry lad I have important things to do we will talk another time." I need to speak to him for the rewards for my quest and to simply add the questline finish to my stats.
Also, side note, I used console commands to finish this quest because Delvin simply wouldn't give me any Markarth jobs.
Best Answer
In particular, you must restore the guild state first before being appointed leader. It always works for me and you don't need to use the console command to complete the quest. Do continuous jobs from Delvin and Vex, until all the merchants in Ragged Flagon back full. I know it's so tiring, but, there is a mod that governs this job. If I were you, I would be patient and don't want to try such a mod because it contains sensitive scripts that can damage quests in the game.
The problem of not accepting jobs to Markath may be that you use mod as I mentioned earlier, and it is damaged. If you don't use mod, be patient a bit. A few tips, always save before you start a job from Delvin and Vex then try a job that takes place together. Complete their job and save again, and so on.