If you cook the oranges for about an hour in the right amount of water you’d use for the recipe and leave them to cool (I do mine the night before), you can then cut them in half and squeeze the juice/remove the pulp easily and the rinds are then soft to slice with a sharp knife or cut with scissors. It obviously makes the process quicker the next day.
The oranges sort of implode a bit, but don’t worry, they’re fine. I can’t remember if its a Delia or Mary Berry hint, but it works wonderfully.
I’m another to cut down on the amount of sugar - by about a quarter.