When creating custom civilizations, I recommend modifying an existing civilization instead of trying to create one from scratch. That’s essentially what the Civilization Builder does when it generates custom civilizations.
Yes, it should be possible to enable more than 4 units. Take a look at the Train Button stuff in AGE.
In my experience, if you have two units/techs sharing the same button, the game won’t crash, but you’ll only be able to press one of the button.
The official Age of Empires Discord server has a channel for AoE2 modding:
The Citadel is another modding Discord server:
You can also my post on creating custom civilizations with AGE:
Are you referring to Steam Workshop? Because the Definitive Edition, unlike the HD Edition, does not use Steam Workshop for mods.