Eternal, actually yeah, we might want to tsunami be used as fast as ferns. Not tsunami speed itself, but only cast.
You see, Sand user has only jutsus which are hard af to hit, like coffin. Have you ever tried to hit running person? Yeah, its possible, but its not that easy.
Back to topic, if you use tsunami and miss, from beggining, you are doomed. Kaguya or Yuki will just stand in a place and hold G. GZ, Sand is dead.
If not decrease cast time, maybe add like armor for it, like 60% for example.