I had a similar question once, not sure if this will help you or not. “At the beginning of the scene” happens only once at the beginning of the scene, since “at the beginning of the scene” can only be true once. Using “trigger once” and “at the beginning of the scene” together is redundant.
“Trigger once” is dependent on a condition (meaning he actions will happen only once each time the condition is true). So, if you have events that activate only once in a scene and you want them to happen at some point other than at the beginning, then you would have to have them activate based on conditions at some point in the scene. Just make sure that condition can only be true once.
Could you elaborate on what the events you’re wanting to happen only once are, and at what point in the scene you’re wanting them to happen?