Maul is only fan service if he isn’t setting up a future film.
That’s not how that works.
If Maul is there just bc he’s that guy from TPM-> fan service
If Maul is there bc he’s important to this off-shoot film series going forward->not fan service
It’s not just that he’s there, it’s how he’s there. A future film could potentially use him in a way that’s relevant to the story and the character that goes beyond servicing fan desire for more Maul, but there’s nothing about the story of Solo that requires his presence, let alone a 2 minute scene, completely irrelevant to the rest of the film and not necessary for future films, where he ignites his lightsaber(?), of all things.
I also don’t believe that because something is set up doesn’t make it fan service. There’s a good argument that set up is a subset of fan service. The Jabba is both fan service and set up, though thankfully in a way that doesn’t detract from the film, like Maul does.