This is just one person's memory/ancedote. Not to be taken as gospel. From what I remember, WB had seen some success with the Teen Titans shorts. WB was thinking Teen Titans could possibly be another tentpole franchise for them, which means a franchise that will always be in production (Scooby Doo, Tom & Jerry, Looney Tunes, Batman, etc). WB looked around at the cartoon landscape at the time, and noted that the most popular series were Adventure Time and Regular Show. It was decided that a new Teen Titans cartoon could be a comedy, focusing on the domestic life of teenage superheroes.
Aaron Horvath had finished up a successful run on the MAD Magazine series, and was given the chance to be the showrunner for Teen Titans Go. Michael Jelenic was an in-house writer for WB, having just finished on the 2011 Thundercats revival, and other series. Michael and Aaron were teamed up, Michael to write, and Aaron to run the show. Eventually, over time, Aaron became a full time writing parter with Michael, and both would oversee and rewrite scripts for freelance writers.
An extremely talented and dedicated crew of producers, directors, storyboard artists, designers, animators, editors, compositors, technical directors, production members, actors, and dozens more make TTG.
The writing on Season 6 roughly started at the end of 2018, beginning of 2019.