The material was already written and just needed to be adapted, as well as a change of leadership midstream for DSC.
I totally understand it's way more challenging to develop characters (and plots) if you're not working from source material, but developing things on the fly. This is why I've said it was a very, very bad idea for Discovery to "go big" right out of the gate. The first season should have been spent developing characters and building tension, with something like the Klingon War only starting at the very end of the season. Good serialized TV after all not only has seasonal arcs, but an arc which carries across the entire show, which means season 1 is pretty much "setting the table" and getting people intrigued as to what is yet to come. Basically, don't shoot your wad all at once. I mean, the destruction of the entire multiverse was threatened at one point during the first season. Everything is a friggin climb down from that.