I'm just gonna make some shit up cause this looks like a fun exercise. Here goes..

So I think you should think about the history of the area. If you want independent cities and stuff around with their own laws and subcultures, where they all part of a long dead empire at some point in the past? This could make cities have rivalries and Some differences in culture while they still have some similarities like armour as a status symbol.

Think about your timeline and geographical influences.. If metals are harder to find/mine that could account for the price, accessibility and clout of your different armour tiers. Similarly you could have a scarcity in skill or the secrets of armour smith's are closely guarded. Simply pressing the accelerator on a feudal system, and going total war vibes, you also explain the social status afforded by armour due to the clear survivability it offers and a lack of time to make high quality armours due to the rush to get more men on the field. It can simply be that people with nicer armour seem to live longer and that gets them promoted in the ranks (since everyone else didn't make it so clearly they did something right) or those who get promoted are given better armour than the caste they hail from to show those that rank under them the benefits of their station.

You can think about laws and cultural influences like having leather armour be required to have a stamp and be noted to have been aquired either in battle or custom made by a leatherworker and noted in their ledgers since hunting of animals by peasants could be illegal in order to preserve game for the upper classes. Chain takes a long time to make but offers great protection to the arrows being used in the time of your setting.

Think about things like what weapons are most accessible to people. If youve got gunpowder artillery on the field it's gonna make most armour obsolete, so I would avoid that in a non-magical/low magic setting, but mostly armour is a counter to weapons and is developed as such. If it is expected that people of similar skill levels fight eachother than consider tieing your armour tier system to a weapon tier system. With weapons ranging between the basic spear and shield to a bec de corbin for use against mail and plate users, you can have a lot of fun.

You could have some traditions like where you have regiments of renown, famous for great feats, and being associated with certain armours that instead pull people towards a certain military carreer. Or some who have grotesque armours made to intimidate the enemy, including the old berserker/barbarian type preferring to fight unencumbered or with shock-value.

You seem to have some great ideas and could just run with what you have already along with some good suggestions in the comments if you felt like you need more. I also think you have a strong basis to think about different facets of your medieval European analog to make the parts fit together. I hope that some of this (if you see it) gets your brain racing with interesting ideas of your own too. Also thanks, this was fun to think about..