Mm. It's already a stretch that Ala Mhigo and Doma were liberated so easily - but that, at least, can largely be attributed to Zenos letting them be liberated easily. As it stands, though, a full on war with Garlemald should be something that the Warrior of Light and Scions seek to avoid at all cost. An incredible amount of people will perish on either side should it occur - and that's not including those who happen to be caught up in the middle of it all. Furthermore the Warrior of Light and Scions have already seen the bitter consequences of a thousand year long war where Ishgard is concerned.
Doma and Ala Mhigo have been liberated. If they do not attempt diplomacy at this point then they have very little moral high ground to stand on and will simply be falling prey to the same bitter hatred that consumed Ishgard. It'll merely be a case of replacing 'Dravanians' with 'Garleans'.
I'd be all for working with someone like Larsa who values peace between Eorzea and Garlemald. Larsa is particularly great since he remained loyal to his homeland and people. He did not defect, he genuinely wanted peace and diplomacy to be embraced whilst putting an end to the rot that existed not only within his own homeland but outside of it as well. Applying that to FFXIV, we've already seen countless examples of rotten figures within Eorzea and Othard but we've also seen numerous examples of good people along the way. It'll be no different with Garlemald - I have no doubt in that regard. The Warrior of Light should have little doubt either since he's encountered the likes of Regula and Baut who are wholly reasonable, honourable individuals.
Which means a full on war is not the way to go at all.
As far as Garlemald itself goes...I do get 'Midgar' vibes from what little we've seen of the place on the official world map. I also suspect that the capital itself will be much more advanced than the military fortifications we've seen in foreign territories. With that said, though, even they're pretty advanced - and whilst it's not as advanced as the Allagan stuff we've seen in the likes of Azyz Lla it's pretty close.