Perfect idea and great multi-use of your jam! Your plan is perfect...just create a nice sweet tart balance to your liking. If you want a little more texture you could plump up some golden or dark raisins by nuking them in the microwave before adding. Coriander seed is often used and provides a little texture contrast as well. A bit of ground allspice could be good as well.
On the contrary, most of the older recipes that I've seen for jams use more sugar (60g per 100g or higher is common) than is popular in some of the newer products, because sugar has a preservative effect. But most likely pre-refrigeration and pre-iceboxes you simply wouldn't have kept multiple jars open at the same time; you'd use one at a time as quickly as practical, and keep your reserve stock seals unbroken.
Industrially refined pectin has actually reduced the amount of sugar necessary to make a good preserve. A number of companies also now make "all fruit" spreads, but they're controlling the sugar ratio with knowledge from food science, by analyzing how much sugar is in the non-primary fruits like grape and apple juice.
60 grams of fruit per 100g or more are available but by traditional standards are quite a luxury, even though I would agree that such products often taste better; they owe their existence to better food science, however, and a little to refrigeration.
A lot of widely-distributed brands still use sugar ratios that are pretty high, including Welch's and Smucker's, but you may find higher fruit ratios in boutique brands.
Best Answer
It sounds like it is oxidizing so you need an antioxidant for the jam. First I need to know what ingredients Greengage jam contains. Then I can advise you on natural antioxidants to add (like lemon juice for avocados).