Hide Next button until objects have been visited

Change the trigger that sets Next to normal.

I know when you use the wizard that "State of..." is one of the options for the When part of the trigger, but it doesn't work. There is nothing to make it happen. States do not initiate triggers. They are very useful in conditions.

Write your trigger to change Next to Normal when the timeline starts if all those objects are in their desired state.

FWIW: You do not need to write a trigger to set the state to Visited, the built-in visited state does that for you.  That said, "Visited"  state is not always stable, especially if you visit other slides, and especially if they have branching on them. I prefer to use a custom state, like "Completed". If you do that, then you will need a trigger to set it.