We know from RQG that closed or open helmets are most common for nobles and warriors (Light Infantry is associated with composite helmets).
So let’s think about styles, starting with Kethaela. Remember during the Inhuman Occupation there was no communication between the humans of Peloria and Kethaela. In the early Third Age, Kethaela was a lot more rural and a lot poorer than it had been during the Second Age. The Closing and the Inhuman Occupation made it a backwater.
Kethaela had access to the dwarfs of Gemborg, so we continue to have access to high quality bronze working, but we don’t have a lot to trade with the dwarfs until the Trader Princes establish a route between Kethaela and Safelster. So good gear gets concentrated into a small elite. But it might be really fancy – think like Villanovan culture in Italy or Urnfield in Central Europe.
The original Kethaelan settlers of Dragon Pass were farmers and herders. Their bronze working was probably pretty simple, so we have a basic bronze cap like a Kegelhelm.
Peloria is different. The Carmanian Empire had a tradition of good bronzeworking, Third Eye Blue, and the dwarfs of the Brass Mountains. Dara Happa had a tradition of heavily armored phalanx warfare going back to the First Age. The Orlanthi remnants in the Pelorian hills had access to these crafters as well. The Lunar Empire took all of this, and added lots of Pentan cavalry. By 1300, the Lunar Empire dominated all of lowland Peloria, and from 1309-1347 it campaigned against the Orlanthi of southern Peloria.
So our Pelorian settlers in Tarsh came with the Pelorian tradition of bronze working. And they warred with the Lunar Empire continuously, had access to Lunar gear and equipment, and were familiar with their tactics.
Things changed when Sartar unified the eastern tribes and built his roads. Trade meant lots of regular contact and interaction between Kethaela and Peloria. It also meant wealth. Crafters settled in Sartar’s cities, and they had access to dwarf techniques – which Saronil later stole from the dwarfs. And pretty quickly, Sartar found itself at war with the Lunar Empire.
With the Opening of the Seas, this got super-charged. Kethaela became incredibly rich, and Sartar as well. Pelorian designs were taken by the Sartarites and localized, even improved upon. Meanwhile, Kethaela kept to more traditional styles but they became more ornate and fancy. And ideas from overseas got incorporated into designs.
For the last century or so, the most common Lunar closed helmet is something resembling a Chalcidian helmet. In Dragon Pass, a variant that resembles a Thracian helmet is in vogue with the nobility of both Tarsh and Sartar. But in Esrolia you are much more likely to see something more decorative (although among the professional mercenaries, the Thracian helmet is going to be pretty common).