it feels like fanfiction, but honestly i don't mind it at all. When you have the budget and the audience to make your fanfic into TV series then why not?
I liked the first season but was sad on how the Pratchett like things where gone or downplayed (the hell nuns, the children, the 4 to 5 horsemen etc.) It clearly was a different book in Gaimans eyes. The acting really did a lot of heavy lifting.
The second season.... I like the characters, mostly. The weird pairing off of characters was just ham fisted and bad. I think you nailed it on calling it fanfiction. Now I will likely watch season 3, but I don't have to like it.
Just here to pile on. Without saying too much, the first romance subplot felt ham fisted and the second was... Ugh. I know I sound reactionary as hell but seriously not the characters for it.
Its not strictly a new story. They based it on notes that Gaiman and Terry Pratchet had compiled for an unreleased sequel. Also, Gaiman has been the lead creative designer on the TV series, so at the absolute worst you could claim its half an original story.