I feel like you really are not grasping racism as a concept. if Sephiran tried to do what you're describing...what do you think that would cause?He even tried to get Goldoa to aid the enslaved Laguz in the beginning but was denied. If Sephiran used his position in Bengion with Sanaki to start a literal Civil War in what is, by far, the LARGEST empire in all of Telius the same thing would happen
edit: Also, no, Ashera waking up has less to do with Kurth getting involved and everything to do with Michaiah, as the inheritor of Altina and Sephiran's union, and possessed by Yune, singing the Galdr of Release
This isn't about racism. The game makes it abundantly clear that as long as one party, the Dragons, stay out of it, Ashera won't wake up. At that point in time, Daein, still ruled by the former king, probably wouldn't join in either as they had little reason to ally or fight with Begnion. Even if it WOULD'VE waked Ashera up, why not try and help the Laguz take down Begnion anyway and see if they aren't able to topple Begnion before Ashera wakes up? Since Sephiran intended her to wake up anyway, why not first try to take down the corrupt Begnion senate? Not to mention, covert methods like poisoning or a random massacre by a masked assassin.
There were countless easier and more logical options other than the crazy convoluted one he came up with. Your apologism for him is like someone from the thanosdidnothingwrong crowd.
Your edit is also meaningless. Yune had to be woken up then or Ashera would've killed the whole world, not just turned them to stone. Plus that proves my point that even with the whole world at war there needs to be a significant amount of casualties before Ashera wakes up, casualties that would be a lot more limited if it was just the world vs Begnion.