Well it wasn't totally his own original story. His "The Sandman" is the third(?) DC character to be called The Sandman. He will likely have had some restrictions and constraints put on him by DC at the time, as it was in the mid-late 80's, particularly when starting off a new series of comics where much was unknown. But this wasn't totally in isolation either. The first "book" or story arc, is kind of a horror spin off from Batman and Justice League. It featured existing DC characters, like Dr Crane/Scarecrow, John Dee/Doctor Destiny, John Constantine (and in later comics he created an ancestor of Constentines, Lady Johanna Constentine), J'on J'onzz/Martian Manhunter, Cain and Abel, the Satanic Triumvirate, a character who is (hinted as being) Matt Cable after he died, and probably more that I don't know. But as C Kent from Krypton, you probably know a lot more DC than I do.
If you've not read it, and you've not watched it (yet), why do you have any worries or questions about how diversely it was or was not written, or how characters were originally written or drawn? Wouldn't it make more sense to read it yourself, watch it yourself, and make your own judgement?
But, if Gaiman was as diverse as you claim why did he not write character xyz as a minority?
If and when you read the entire run, you will find many times he did. Far more than was in most other media, from film, novels, TV, and other comics of the time.
I hope you do enjoy it when you watch it. Yes, some things have changed from the books. The authors own experiences in life will change his point of view in 30 years since it was written. I'm not the same person with the same points of view as I was 30 years ago, otherwise I'd be here saying how brilliant Starscream (Transformer)was. The most recent Sandman book/collection that I've read, Overture, has a bit of a different perspective on family, parents and children, than the original 75 issue run. But Gaiman is a parent himself now, and time and life changes you.
The whole story of the Sandman is about stories, family, change, responsibility and accountabilty. I started reading them at uni over 20 years ago, and I still think it's one of the most incredible things I've read.