Dynamic Maze Navigator

You are given a 2D grid representing a maze where each cell is either open (represented by '.') or blocked (represented by '#'). In addition to static obstacles, certain cells in the grid are dynamic: their state toggles between open and blocked every turn. The schedule for each dynamic cell is provided: an integer indicating that the cell toggles its state after every N moves (N ≥ 1). All non-dynamic cells remain constant.

The maze also has a designated start position and a target position. Your task is to write a program that computes the minimum number of moves required to reach the target from the start, obeying these rules:

1. At each move, you can travel to one of the four adjacent cells (up, down, left, or right) if the destination cell is open at the time of arrival.
2. After each move (including the initial state before any move), update the state of each dynamic cell if the number of moves so far is a multiple of its toggle interval.
3. If the target is unreachable, the program should indicate that no valid path exists.

Assume the following:

• The grid dimensions, start, and target positions are provided as input.
• The list of dynamic cells with their positions and toggle intervals is also provided along with the initial grid configuration.
• The grid indices start at 0.

Input Example (you can choose your own input format):

• An integer for grid rows and columns.
• A grid of characters ('.' or '#') describing the maze.
• The starting coordinate (row, column).
• The target coordinate (row, column).
• A list of tuples, each containing the row, column, and toggle interval for a dynamic cell.

Output Example:

• The minimum number of moves to reach the target, or a message indicating that the target is unreachable.

Design your solution with efficiency in mind, as the grid can be relatively large and the number of dynamic cells non-trivial. Good luck!