Question RTS game help - Terrain and Environment manipulation during Runtime

I have asked the same question in UE forums but I don't seem to be able to get an answer there.
My initial choice was UE because of blueprints (and new graphics capabilities) but it seems that UE is incapable of terrain manipulation after runtime, at least no one is answering my question.

In that case, if I have to delve into programming then my first choice is Unity.

So here is my question.

I am in the process of creating a strategy game. It will be a top-down view, maybe isometric, and most likely in 3D.
Part of the strategy in the game involves manipulating the terrain to gain an advantage, which means, creating hills, valleys, mountains, and even water bodies.

The game is not real-time multiplayer, so the changes can be done to the terrain at say a separate stage of the game (if needed the game can be split into phases, like having a building/consolidation phase, before the fighting/action phase).

So the game plays kind of like an RTS and Card Game (MTG) fused together. The player is a wizard that has the power to manipulate nature in a limited manner. The player can create slopes, hills, rivers, forests, etc. to slow down the enemy or more impassable terrain like mountains, cliffs, and lakes. Except that some enemies like flying units will not be affected by mountains or lakes, and some amphibious units (like lizardmen) will BE faster on water bodies. So, the enemy units have to be able to pass through the terrain. (To put this into context, I thought of manipulating terrains, like adding hills or mountains, as if I was adding a structure/building, but then thought that it would become a barrier and prevent any units from moving through)

To make the gameplay more simple, the map area will be divided into grids.

Can anyone suggest a "Visual Script" or "Market Resource" or helpful resources on how to create editable terrain and environments during runtime?