Prequel implies it was created after the original but takes place before it. While Peter Jackson may have made his movies in that order it is not how Tolkien wrote them. Calling the movie a prequel I guess works but not the books.

I very well could be wrong about the time period, I was 100% certain Trump would never be president, but I don't think it is an accident that they specified it will take place before TFOTR instead of saying takes place before the Hobbit the original books. If you want to stick with the most popular characters it is the time frame to use. Tolkien only hints at what was happening in that time period but there is a lot meat for epic fantasy there with a ton of room to expand it in ways that would be true to the story line and still give you epic fantasy.

The Sillmarillon on the other hand would need to either be focused on a few stories, and then you are back to 95% filler, or is would have to simplified and basically guttted because it is was to complicated with too many characters and covers to much time. A lot of the primary actors in the story do not even show up until deep in the story. I would love to be wrong, assuming they do it right, but I will almost as surprised as I was when Trump won. Most fans will tune into that first episode looking to see the characters they know and love not a bunch of gods singing to each other. Maybe they can bring those stories in piece meal as flash backs like Jackson did for the Battle of the Last Alliance. To be any good though, and by their standard that means competes with GOT it needs to have the Epic plot line where the fan of the world is in the balance, not sure how they will do that.