From what I understand, Oban is not that interesting but makes sense if you are visiting Mull for a day. We didn’t go there but that is what I was told while planning my trip.
Perhaps unsurprisingly a lot of people would disagree with that comment, especially if they have actually been there.... Oban regularly features on itineraries and for good reason. Not only is there the town to explore with a walk up to McCaig's Tower for the views. There is Kilmartin Glen just a short drive with perhaps more prehistoric sites in a small area than elsewhere in Scotland. There is Mull with Iona and all its early Christian history. Alternatively there are the islands of Kerrera or Lismore - see isn31c's suggestions for them on this thread
Glencoe is always popular, especially for those wanting to walk and there is some serious walking you can de from there.
BUT I'm not sure that I would replace Oban for it. You will see stunning mountain scenery on Mull if you do a day trip there and also on Skye. If adding it, then something else would have to give.
The logical placce would be between Oban and Skye but that throws the rest of the itinerary out. The other alternative would be to take out Pitlochry and rejig dates...
In the time you have available Linda, difficult decisions do have to be taken as you can't see everything and itineraries do need to be kept realistic. Yours is and works. It may not be what others may choose but the danger of a forum like this is that everyone keeps throwing in ideas and if not careful you end up with a camel (a horse designed by a committee of experts...