Start on "Hero".

I didn't choose water, but I created a map with it because I really don't like these sharply divided biomes (like, we have snow around the city, we break through the neutrals, and it's summer).

Well, you have to put up with it. Here's a good channel for beginners on how to play on "Hero" difficulty with companions: https://www.youtube.com/channel/UCUuspYnSQHiac_AKeXp7Etg And yes: neutrals are easier to defeat on "Hero" than on Veteran or Warrior, assuming the same army (or rather, fewer losses). This is because on "Hero" difficulty, more modes are enabled, such as the "danger" mode, where the enemy doesn't position itself to be attacked by our units, which gives us additional advantages and turns in the game.