Total War: THREE KINGDOMS

If playing as a Han faction, you will unlock a reform once every 5 turns (every spring). If you're playing as Cao Cao, he has a scheme that will instantly unlock a reform, which can be used once every 20 turns (don't use this scheme during spring, or at least don't use it before unlocking the normal reform, as it may count for both and you'll only unlock one reform that turn instead of 2). This will take you a very long while. It is likely you'll beat the game and even control the entire map before you unlock this achievement. I recommend at least making every remaining character a ally or vassal so that you can skip turns and not worry about fighting wars waiting for this to unlock (check the faction leader's personality in the diplomacy window. Some people are treacherous and more likely to break deals, so it may be easier to simply get rid of those people instead of allying with them). Similarly, making sure all of your commanderies' public order is positive will keep them from spawning rebellions. Public order seems to be primarily affected by the population size of a city, so you can downgrade cities to have a lower max population. There are several buildings that affect both population size/growth and public order, and you can also station an army inside a county to help keep the peace. Some ancillaries and faction council actions will also help you control public order. You can hover over the public order of a commandery to see what elements are affecting it.

You should also have unlocked Branching Out during this.

Make sure to also get Forbidden City if you haven't already (the palace is locked behind yellow reforms for most factions).

Yellow Turban, Bandit, and Nanman factions have their own reform systems with their own quirks including variable research rates, so you may have better or worse mileage with those factions depending. This achievement should be possible with Yellow Turban and Bandit factions, but I'm not sure about Nanman as they have a lot of mutually exclusive reforms. Serious Trivia has pretty good video overviews of the different reform systems if you want to preview how they work before trying to play them.

Bandit Reforms - Yellow Turban Reforms - Nanman Reforms -