How can I add a text to base layer once all layers are visited?

Set the Initial state of the text to Hidden.

Change the state of the text to Normal after the layers have been visited. The trigger(s) to use depend on how the slide is set up.

For example, give the buttons that show the layers a Visited state. An object with a Visited state will automatically change to that state when clicked. So change the text to Normal when all the buttons are Visited.

Otherwise, to ensure that each layer has been completed, you could change the corresponding button on the base to a custom state when the layer's timeline ends, and then change the text to Normal when the buttons are all in that custom state.

You could also track layer completion with variables.