It looks like the silver shovel unlocks the Waterscaping Permit and the golden shovel unlocks Cliff Construction Permit
So I think it'll take about as long as it took in previous games to get these tools. We will need to test out the burial vs. fertilizer methods (if there is fertilizer) for the golden one and see if fossil donation gets the silver one still. They may change the requirements to get these tools though!
Considering we are being allowed to customise our island how we want and they even mention in the direct that it?s up to us if we want a bustling urban area or a deserted island I think it might be something we get to choose. Like an option at resident services where we go ?Yep the island is done now?. So it will entirely depend on how much we want done to our towns. As I?m typing this I wonder if that means we won?t be able to build anything after that point - like it caps the PWPs or something.