A longer process time? Not sure what you mean by that.

Peaches are low-acid and low-pectin so getting them to set well with no commercial pectin and reduced sugar is not likely. You can in theory cook the jam long enough to evaporate out sufficient water for a thicker mixture but it's not truly jelled and there's a risk of over-cooking and the loss of freshness of taste.

The set of a jam requires a specific ratio of acid+sugar+pectin. So in a sense you're asking mother nature to break her own rules. While some reduction of sugar is possible it's likely the best result will be a softer set preserve.

The other option would be to improve the setting qualities by mixing the peaches with a higher-pectin fruit or fruit juice, like apple. You can, in fact, make your own natural pectin base from apples or citrus and then use it in your jams.

Carol

Try Melly's Peach Vanilla Jam a la Ferber. I don't know how low-sugar you're trying to make it, but it does thicken using the macerate, boil, then let stand method.