You say written math was never developed. OK, that’s cool. There have been lots of illiterate societies that made things work for a long time. People still do math-y things, even if they don’t know what the word “hypotenuse” means.
I’d look at known illiterate societies in human history for inspiration. May I suggest Iron age, and Early Medieval Ireland? Not the monasteries, mind – those were built by literate priests who gained knowledge from study with Rome – but the villages.
There are no urban areas. Homes are rough-shod, but tribal chieftains were able to build entirely synthetic islands on which to live (the Google word there is crannog). There’s no known intricate architecture outside of the monasteries, but damn if they didn’t make some fine goldwork
They had a goods-based economy, so no fiat currency. Most things’ value was based on an equivalent value of a cow.
They had an intricate legal system that functioned without “police” to enforce it. If you were found guilty of causing harm to a party, your family could be sentenced to pay to the victim’s family a certain number of cows, based on the victim’s honor price. The honor price was set based on a person’s status in the also-intricate caste system.
An important feature of their civilisation is the filidh class, whose job was essentially to memorize the law and recite and interpret it at will. Knowledge was passed from one person to another by rote, instead of by writ.
So, there’s plenty of civilisation to be found in an illiterate society. It’s just harder for historians to study cultures that don’t write shit down, and that is why we don’t hear much about them in high school.