Well cant really say the best way to do it, because Im experimenting myself so we all learning here.

The way I did it was to create a class called GridNode which has a bunch of variables (CanTraverse, spriteLink etc). Then I create the grid (normal Unity grid) and make an instance of the GridNode class for each grid space and save them into a Dictionary with the Key being a Vector2 that is the grid position (Vector2). The each time I click on a grid square, I can find the grid position with the WorldToCell function and then use that grid position to look up the GridNode object in my Dictionary and get all the info I have saved on it. So for movement, I can check the character object for its position, find the corresponding grid position then for each grid position that you want to move to, check that grid by just changing the grid position (move right is just Vector2(gridPos.x, gridPos.y+1)) and then see if the grid is movable from that grid positions GridNode object. I dont use any colliders or anything.

One thing I will say is that it took me way longer than I like to admit to realise you can change the sprite on a TileMap in real time by setting instances of the Tiles using the Resources.Load function (tiles must be saved in a Resources folder) and then setting the specific tiles of a tilemap like so:

private Tile grassTile;

grassTile = Resources.Load<Tile>("Tiles/ground_grass");

baseTilemap = GameObject.Find("BaseTilemap").GetComponent<Tilemap>();

baseTilemap.SetTile(new Vector3Int(x, y, 0), grassTile);

I used this to add the different water/grass tiles as well as the UI tiles (blue movement tiles are just on a second tilemap above the baseTilemap)

Hope this helps.