Is there a way to set and change the transparency of a complete layer?

You'd have to set the transparency of all the items on those layers. Here's a script that does that.

At lines 3-5 you include the names of the layers that should be targeted.

Then at 'opacity' you set the desired opacity. To return to 'normal', set it to 100.

(function () {
  
  const layers = [
    'Construction-Paths',
    'Bilder',
    'Texte',
  ];
  
  const opacity = 50;
  
  
  function setOpacity (layer) {
    if (!layer.isValid) {
      return;
    }
    var items = layer.pageItems.everyItem().getElements();
    for (var i = 0; i < items.length; i++) {
      try {
        items[i].transparencySettings.blendingSettings.opacity = opacity;
      } catch (_) {
      }
    }
  }
        
  
  for (var i = 0; i < layers.length; i++) {
    setOpacity (app.documents[0].layers.item(layers[i]));
  }

}());