For what it's worth, the casus belli given by Sylvanas to Saurfang is... actually pretty sympathetic.
The events of Before the Storm, notably The Gathering which you just pointed out. A "peaceful meeting" shot up by Sylvanas because Anduin brought a usurper who, in the moment, really did try something that was not agreed upon by Anduin or Sylvanas. Sylvanas' actions went too far for Anduin but she had the "legal leeway" to do it. Anduin explicitly says he can't even call her out on it.
The first events of A Good War or Elegy, notably the fact that Anduin has become far more pushing towards the Horde. Not aggressive per se, but pushing: the amount of spies in Orgrimmar makes it sound like the place is about to flood with them. This increasing tension with the advent of Azerite, among a couple of other reasons is what enables Sylvanas to convince Saurfang that whatever tenuous peace she and the boy-king has (even after "she humbled him;" AKA Saurfang probably not knowing what really happened at The Gathering but knowing it pissed a teenage king off), it's not going to be permanent. Other reasons included Genn's attack on Sylvanas in Legion (Stormheim), which was a notable reason because "What if the peace-loving boy-king can't keep his dog leashed?", and also the history of hatred between the Alliance and Horde: orc internment camps, prejudice against blood elves (notably WCIII), and Daelin Proudmoore's campaign against orc and troll alike are all good examples.
Cue the rest of A Good War, where Saurfang executes a plan to drive a wedge into the Alliance and divide them, which would enable the Horde to "carve out a peace with each individual nation." Holding Teldrassil and more importantly its inhabitants hostage was key to this plan, by the way.
Sylvanas throws it out the window and fucks it up by burning the World Tree, discarding a valuable bargaining chip (the innocent lives at stake) and all but ensuring an all-out war between the Horde and Alliance.
Step 4 is the most important bit. Sylvanas' reason for war as given to Saurfang was clearly not her reason for war, and we see that increasingly so as we progress through BfA. She was never out for peace. Heck, she acknowledges with Nathanos that she has longed for years to conquer Stormwind.
Peace isn't her objective. It never was. And it irritates me that Saurfang played into her hands and decided to start the War of the Thorns so willingly. Even more so, he ended up seeing her point when she used some bullshit logic at the end of A Good War to justify the genocide.
Which basically means that the Battle for Azeroth TL;DR above is bad writing. So the foundation of BfA is bad writing. That absolutely sucks, because it diminishes everything in it and what comes after it.
There are other reasons I'm unhappy with Battle for Azeroth's plot and storytelling as well, but by far the most egregious is how they've turned Sylvanas from a cold, cunning, and calculating agent into a brazen and stupid warmonger.
And what's worse is they may play the "4D Chess" card and we've played into Sylvanas' hands all along. In which case, congratulations Blizzard: your horrendous storytelling kept me less interested in the lore than I would have been if you properly hinted that Sylvanas isn't a complete moron. Because right now you haven't shown that. It's a 50/50 whether you've just driven her right off the rails, or whether you've just done a piss poor job showing that she isn't right off the rails. Neither is good.