I was able to make my own functions to convert from offset coordinates (what unity tilemap and SimplePathFinding2d use) to cube coordinates (what I presumably need to be able to get hex ranges and line of sight). I replaced the square range finding for highlighting moves (which was complicated and distorted every other column you moved) for this hex one- this is currently just moving out in each direction in a star shape, but with a little more puzzling I should be able to get the whole range (I hope).

For anyone else who needs to convert from default Unity Tilemap offset coordinates to Cube, hereโ€™s my functions. Refer to the redblobs site if you need to do flat-topped hexagons,

    public static Vector3Int CubeToOffset(Vector3Int cube)
    {
        int col = cube.x + (cube.z - (cube.z & 1)) / 2;
        int row = cube.z;
        return new Vector3Int(col, row, 0);
    }

    public static Vector3Int OffsetToCube(Vector3Int offset)
    {
        int x = offset.x - (offset.y - (offset.y & 1)) / 2;
        int z = offset.y;
        int y = -x - z;
        if (x + y + z != 0)
        {
            Debug.Log("the sum of cube vectors must always be zero!");
        }
        return new Vector3Int(x, y, z);
    }

update and now full hex ranges work. Hereโ€™s how I am getting the tile range to visualize where you can move -

    Vector3Int PiecePosition = overlayTilemap.WorldToCell(transform.position);

    Vector3Int CubeCoords = HexCoordinates.OffsetToCube(PiecePosition); 
    
    int x = CubeCoords.x;
    int y = CubeCoords.y;
    int z = CubeCoords.z;
    // I love constantly fixing range by one
    int adjustedRange = range - 1;
    for (int i = -adjustedRange; i <= adjustedRange; i++)
    {
        for (int j = -adjustedRange; j<= adjustedRange; j++)
        {
            for (int k = -adjustedRange; k <= adjustedRange; k++)
            {
                if ( i + j + k == 0 )
                {
                    overlayTilemap.SetTile(HexCoordinates.CubeToOffset(new Vector3Int(x + i, y + j, z + k)), highlight);
                }
                
            }

        }
    }