I created a separate mod that handles all the new units I've added to the game that I want to use as city garrisons, though I imagine you could add it to your own new unit mod's artdef w/o creating a new mod. You'll just need to copy over the section under the "UnitGlobals" collection and then change the "GarrisonDefenseUnit" "m_ElementName text" to whatever your unit is defined as - "UNIT_XXXX" under "ERA_RENAISSANCE".
That's something new that was added with the update accompanying the Gathering Storm release. I didn't look at it too closely, but both regular and "CITY_DEFENDER" units appear to function identically and utilize the same elements, so I'm unsure what the purpose was behind the change. In any event, you can enter your unit as it is defined and it will work properly.
Not sure what file you're referring to here. The garrison unit's (i.e. city's) attack strength and defense strength are calculated based on the regular unit stats of units you've built and whether they're currently stationed in the city, whatever they might be. The actual unit graphic used for the garrison is based on era and is visual/cosmetic only. It has no effect on the city's stats.
That is partly true. Because if you start the game in a later Era, the ranged strength starts at a preset level. For e.g. it's 25 in the Medieval Era, even if you lower ranged combat strength of Archers.
It's a bit different if you start in later eras since there has to be a minimum value for city defense. If you want to modify that then you need to change the values per era in the 'StartingRangedStrengthMajor' and 'StartingRangedStrengthMinor' columns of the 'StartEras' table. These values currently correspond to the highest base game ranged attack value of units available (i.e. free units you begin with) at the start of each era. Which makes sense since the city ranged strike value equals the highest ranged attack value of whatever units you've built.
