Your 3rd and certainly your 4th options sound like cheating (well, not cheating, but taking advantage of the fact that its a game).

Maybe try to use rivers and mountain passes to your advantage. A fort in a narrow mountain valley works well, too, without breaking role-play.

Probably the best way is, while technically a cheat, not considered cheating at all: the force diplomacy mod. Depending on your usage of the mod, it can make the game feel more realistic. You should read up on it:
https://forums.totalwar.org/vb/showthread.php?t=80763