>>2081013They buffed population mechanics a lot in patches, making it easier to maintain and giving you better rewards for it.
There are so many sources of public order, if you stack even just some of them you can end up building all your commanderies up to regional cities or bigger with maxed tax rate and still end up with 100 public order everywhere. Ancillaries, character backgrounds and traits should be enough, especially in the emperor stage when you can have six people in addition to your faction leader giving faction-wide effects, but reforms and some faction unique mechanics and buildings help as well.
Once you have big cities, it's really nice how much more income, food and (local) replenishment you get just for having a huge population.