However in the case of DS9 and Voyager, even reading very early novels will spoil the events of the first episodes.
Not really. I'm currently going through the numbered Voyager novels, and while they do talk a lot about the pilot and how Voyager ended up in the Delta Quadrant, the books barely touch on any other episode.
But for Enterprise
, I would suggest that you read the novels prior to seeing the TV series. I was first introduced to Enterprise with the books and I found that when I saw the episodes, the episodes paled in comparison to the books. Enterprise should've been left for the books.