To be fair, just 1 single legion was enough to threaten the existence of an entire continent in ARR, and they were established in a small handful of outposts and had to generate their own resources and supplies, including ceruleum from behind enemy lines.
The Spanish pulled off the same thing when they conquered the New World.
I just don’t think we’re going to get that storyline because it’s already been done before and I hope the writers are better than the ones for The Force Awakens and don’t just copy the same exact plot from an older, successful story and just paste it in with the serial numbers scratched off. And also because I don’t see the value to the Garleans for taking it over since it’s so far away. They could barely come up with a justification for invading Aldenard and Othard, let alone Tural.