No, it was reportedly always the plan to wrap up the war in one season. Indeed, Fuller's original desire was to do a seasonal anthology with each season featuring different characters and situations jumping forward through the Trek timeline.
That's quite routine, except for shows with short seasons or Netflix-type things that drop all at once. And of course plans did change once Fuller was fired and Berg & Harberts took the lead, but that changed things from episode 2 onward, rather than being a late-season change.
Like I keep saying, there's always some "negative fan reaction" to anything new in Trek, and it's usually just a few really loud people on the Internet who egotistically believe they represent the majority when they really, really don't. To hear people online talk about it, you'd think the Kelvin movies were universally hated flops, but in fact the first two were the most financially successful Trek movies in history. So studios and producers should know better than to take that kind of "negative reaction" seriously, because it's always going to be there from a certain portion of the audience no matter what you do. All that matters is the ratings or the box office.
And season 2 is not moving on entirely from the Klingons. We know that L'Rell will be back as a recurring character. The war is over, but the DSC-style Klingons are alive and well.
As I said, I'm trying to be charitable to the writer's room - assuming there were conflicts further up the corporate chain which muddled the narrative. If I instead hold to the belief you outlined - that the ending we saw was what they planned from the beginning - then they really are just a bunch of talentless hacks, because there was so much that just didn't make any narrative, character, or even logical sense.
There absolutely were, however, narrative changes which happened on the fly. For example, apparently Jayne Brook impressed the crew in Lethe, because the decision was made at the last minute to not have her character die. Thus the whole arc with her and L'Rell on the Ship of the Dead, and her later involvement in the final two episodes, was improvised.
Oh, and (looking at Memory Alpha) it seems on the first After Trek it was revealed that not only had the finale not yet been filmed when the opening two-parter premiered, but Harberts was still writing it. This is interesting, because After Trek 1 was produced on September 24th, with filming of the final episode scheduled to start the next day. It was still being shot on October 7th. My understanding is that working on a script so close to filming is very rare, and it can help to explain why the narrative is so sloppy, even if the broad strokes were really intended from the start.