By "the starting point" most people mean the one at the beginning.
In this case, I believe the goal would be to find a starting point to a repeatable forced series of moves that ends in mate. I see a virtually unlimited number of starting points.
You're right, that's why tablebases are built going backwards from mate.
This is the set start point
That is the setup. What is the first move? Let’s just start with a3 for example. Black has 10 possible responses. White has 11 possible responses to those 10. How many moves will be made before reaching a forced series of moves that culminates in mate? Every one of those moves is a start point.
I make it 20 and at least 19 to each of those 20.
By "the starting point" most people mean the one at the beginning.