Maul is only fan service if he isn’t setting up a future film. If he’s just there so people are like “hey, I know that guy” , I get it. But if he’s the big bad in the next film, then it is worth it. If he’s written half as good as he was in Rebels, I’m in.
Only concern I would have would be having Solo interacting with Force users after he is so dismissive of the Force in ANH.