I've been thinking about this for a while (left the relevant text for this answer), since I don't quite like how that's specifically organised (it doesn't make much historical sense, although I know it's a preliminary idea), but something like this could be useful to organise civs and their tech trees better, while facilitating the possible eventual implementation of the mentioned Grand Vision. Now, given that the Dorian invasion hypothesis has fallen out of favour in recent decades, and considering the latest genetic studies (https://www.science.org/content/article/greeks-really-do-have-near-mythical-origins-ancient-dna-reveals), I think something like this would be more accurate (although simplified and still debated):     The issue is, all these transitions happened before 500 BC, so this is not for present gameplay transitions (that's why I removed P1, P2 and P3), unless the time frame is eventually changed. For now it would serve mostly to organise things better, since basically all Greeks would be covered if the Achaeans are included (nice logos for the four tribes would be needed), thus any city-state could be easily implemented from this if needed (for campaign reasons for example, which is how this thread started), each ("hypothetical") transition opening different options.

Had a fast look (since I'm quite busy with other things right now) at The Oxford Handbook of Palmyra, Chapter 12, Palmyra and the Military, From the Roman Period to the Islamic Conquest (that is, "between the first and the early seventh centuries") to have some idea in what units to focus on. I wonder what's the idea of this civilisation anyway, being that they were independent for just a very few years.   Regarding Romans in Palmyra -"The presence of alae is unsurprising in the Palmyrene, as horses would have allowed for the mobility necessary to guard and patrol the vast Syrian steppe". -"The Ala Vocontiorum" "was replaced by the Cohors I Flavia Chalcidenorum Equitata Sagittariorum", which "included a cavalry element, infantrymen, and Palmyrene archers". -"Following the events of 272 and Zenobia's rebellion, a garrison of six hundred archers was left in the city". -"The presence of a legion—which in late antiquity may have numbered some one thousand men—rather than an auxiliary unit, is indicative of the growing importance of Palmyra as a stronghold along the eastern limes". -"Limitanei (frontier troops)" were also present.   Regarding Palmyrans in the Roman army -"It is reasonable to believe that the military skills of Palmyrene soldiers derive directly from a military tradition first developed to protect the caravan trade and that this ‘caravan police’ was later integrated into the Roman army" and "their adaptability to arid conditions played a pivotal part in their recruitment". -Sometimes "Palmyrenes constituted auxiliary troops of their own", an example being the "Ala I Ulpia Dromedariorum Palmyrenorum": dromedary riders. -"Numeri Palmyreni Sagittarii (Palmyrene archers) are known to have been present" at some early Roman campaigns, while some "may well have been Palmyrene militia not in service in the Roman army at that time". -"The Cohors XX Palmyrenorum" had "their headquarters" "in the temple of Azzanathkona", "and included infantry and cavalry". -Towards late antiquity, "the Cuneus Equitum Secundorum Clibanarium Palmyrenorum was a vexillatio comitatensis—a cavalry unit—made up of an elite mounted unit of clibanarii, heavily armoured shock cavalry". A "Ala Octava Palmyrenorum" is also mentioned.   Regarding Palmyrene militia -Native militias had the "role of" protecting "the lucrative caravan trade to the east", and "there are at least three known ways in which soldiers could have served in this militia": 1) To "be recruited by the synodiarch or archemporos to provide caravans" "with military protection". 2) To "actively" combat "the nomadic threat, and served under a ‘strategos of the peace’". 3) To "guard nodal trading posts in Persian territory", and "detachments of horsemen" are mentioned. In any case, and for these points, "their composition and organization remain obscure".   There's plenty of bibliography at the end of the chapter, some of which could include illustrations.

Here one image that I did with AI:

I couldn´t find enough references, but here some: About the infantry, it seems that palmyrian soldiers used conical helmets. Laminar armor, round shields of 90 cm and spear.

I will look for references, mainly from the infantery. Then, I´ll do some sketches.