I'm not convinced that the Shin are the original voidbringers, we learn in the following WoB that the Shin share a language root with Horneaters and Listeners.
I'm writing this post with the assumption that even if the Aimians are not native to Roshar, they are not considered Voidbringers, and have their own special history. We know there are also many mass exodus and migration of peoples in the cosmere that have been alluded too, besides the voidbringers, and there could be a complex political climate that we're unaware of. The Threnodites suffered some kind of tragedy and had an exodus, Iri is on some kind of Holy Journey, the Long Path (and they're at 4 of 7), and the Voidbringers killed their planet. Could Cultivation have offered Roshar as a haven for refugees of the Shard War? The Shin cultivating the land that was given to these refugees as a form of custodians to help them get back on their feet?
The Languages WoB gives us 5 roots, Dawnate most aligns with Cultivation, and Aimian is a crapshoot. Vorin, Makabaki, and Iri are the main 'Human' roots. My pet theory of the Iri are that they are on the path to reform Ambition, collecting cognitive knowledge by living on 7 planets before you've experienced enough to reform your god is an ambitious plan, but that's the only parallel. They could be any shard, but they don't particularly seem Odious except for the fact that they sided with him during current era. Vorin could be the people who destroyed their planet and brought Odium to Roshar, it aligns with their Tranquiline Halls mumbo. Makabaki could be Honor's peoples, who he brought he either to defend Cultivation from Odium, or they were also unable to live on their original planet.
We don't really know if all of the 'humans' came to Roshar at the same time. The Vorins could have came first, as refugees, accidentally bringing Odium. Honor responds with bringing his own troops, the Makabaki. The Iri can show up anytime.
Ohh, and in a bout of even more wild speculation. Each peoples rode upon a moon to the planet.