How mod-able is AOH3?

[FlaWer 4]

Just a question. May I know how far can the game be modded? Aside from the GUI or UI of course as you had confirmed that it cannot be modified. Like the more backroom parts of the game like the combat system or unit functions (How the unit works, like how it moves, AI, etc. Basically how it works but not it's details like prices of recruitment or damage), to the surface level ones like changing the portraits of units or icons of resources to the leaders birth, rule, and death. But my guess is any very deep changes can't be done as UI is already a limitation.