Question about tactics movement

Hello,
I'm working on a tactics mechanic (similar to Fire Emblem or Divinity: Original sin 2)

I've tried two ways to accomplish this:

1. Grid based movement with A* pathfinding

2. Navmesh movement

I'm currently running into an issue with each:

1. Grid Route Due to my world being procedural and not a perfect square, I can't find away to generate a grid automatically that covers the entire floor and stairs that doesn't require me to create millions of tile gameobjects manually. Sure I could generate a X by Y grid but what happens if the room is a weird shape or not perfectly square.

2. Navmesh Route Navmesh works wonders and is easy to setup, however, I can't figure out how to highlight the walkable radius around the player while taking into account the navmesh's pathfinding.

Sorry if these are vague questions but I'm really stuck and I can't figure out which direction to take.

Thank you in advance for any help.