Follow up
Based on the advice of our Dolomites guide we went to the lower lakes about 4pm Sep 27 and the upper lakes at 8am (which is when the boat starts) the next morning. In the afternoon there were still big crowds and loads of people taking selfies - and the light was not very good either. I think pretty nearly all the falls are best with morning light. We had the upper lakes almost to ourselves until about 9:45, and by 10:15 when we finished the trail was getting crowded. The early morning was very pleasant and I got some decent shots, but nothing fantastic. The board walks limit the shooting options and become a nightmare when the crowds arrive.
Apparently UNESCO has told the Park to cut the number of visitors in half or they will withdraw support. That would still be way too many people for me.
Bottom line - if you're passing through it's worth spending a couple hours in the morning - otherwise, it's not worth a trip.