However, I think it's unimmersive (it's dangerous to be the tallest tree.except when you're not the tallest tree, the AI doesn't get the mechanic) and almost removes the diplomacy side of things after RD.

Realm divide in vanilla is a way to give you late game challenge and prevent you from expanding too early and too quickly. I'm not sure you'll like the same, but a few mods I use: For the most part, I mod to tweak things to my liking.