I just watched some explanation / theory videos on YouTube. Darth Maul isn’t a Sith anymore and he is (most likely) the big boss running the Crimson Dawn? No wonder I felt so out of the loop during that cameo! I guess I really need to watch the animated series now.
In the credits the character is just “Maul”
He isn’t a Darth anymore. He gave up the ways of the Sith after the Emperor betrayed him and killed everyone he knew/loved. His main mission in Rebels is to gain power to destroy the Emperor, until he discovers Kenobi is still alive. At some point, he becomes trapped on Malacor for “years”, which is where the Rebels crew meets up with him 3 years before Battle of Yavin. This film ends 10 YBBY, so there are 7 years for them to play out any sequel with Maul involved.