Don’t add all of the lemon juice in at once – you need to add it to taste. It also may seem like a strange ingredient, but it really brings out the flavour of the peaches.
The ratio of fruit to sugar you are looking for to get the right consistency is basically 4:1 fruit to sugar. Work this out by weight. Weigh the sliced & peeled fruit, then divide it by 4, which is the amount of sugar you add.
In this recipe, the sliced peaches weighed 800g, so divide that by 4 and you get 200g.
If you want to scale the recipe up or down, it should still be a good consistency as long as you stick with that ratio of 4:1 fruit to sugar.