It is true that there are a lot of unknowns in how Garlemald operates. But we know how Garlemald approaches its conquered people (non- to second-class citizens, forced conscription), and we know it from an unbiased source; in addition, we know how Garlemald approaches the Eorzean population capable of summoning Primals, and given what went down when they tried to take over Othard, it's hard to see them approaching that situation any differently.*

Whatever the case, the game's moral compass thus far is definitely not pointed in current-Garlemald's direction. Even if we do find out they had good reason to up and conquer Ilsabard and/or Doma, that doesn't change the fact that their actions NOW are oppressive, carried out with little reason other than their own sake (or, more likely, to keep the war economy rolling and to keep up a certain standard of living for the actual Garlean people, at the expense of those conquered; this is how empires operate). In fact, "we see, we conquer" is very much an established Garlean attitude - we outright KNOW that they tried to invade Eorzea because it was there, why assume Othard was any different (in fact, the ever-useful "Rise and Fall of the White Raven", written well before there were Domans kicking around, directly implies this)? Let's not forget that Ishgard still needed reforming and Niddhog still needed slaying, even when the origins of that conflict were revealed to be as muddy as they were; current Garlemald leadership are active antagonists seeking to spread their government to every corner or Hydaelyn Just Because, so no matter how that started, it's a problematic attitude that WILL need to be curbed.

No, if/when we ally with Garlemald, it's likely either going to be reformed ala Ishgard (or at least as part of the process of doing so; this would be so reminiscent of 3.0 that I hope it's not the route taken), or use the morally-dubious nature of such an alliance as a means of showing just how desperate the situation as become. I really don't expect the game to try and absolve Garlemald as it exists now.
__________________________________________________________________________________________________________________________________________________
*I actually have no idea where this "Garlemald lets their territories keep their traditions and culture!" thing comes from, since the only thing said on the matter implies the complete opposite; was it Gaius' 1.0 offer to the Eorzean city-states, which was along those lines?