Best way to preserve fruits is to mush them with sugar & keep in the fridge (our outdoors if it below freezing in winter – it can’t freeze). Very popular in Russia. No cooking, so vitamins are preserved.
Mix 1:1 with sugar for most fruits, put a layer of sugar on top of the jar. You won’t need an airtight lid. It normally keeps OK till next summer. If any mold appears on top – just discard it, the rest of preserve will be fine. For more healthy option you can try to substitute sugar for honey or agave syrup. We do this way our homegrown blackcurrants, white and red currants (last two are better filtered – they make clear jelly – I think its due to high pectin content), kiwi (actinidia), gooseberries, raspberry, rosehip, buckthorn, strawberry, juneberry. Same for wild cranberries, bilberries, lindenberries, dewberries. For the more acidic fruit better to add more sugar (honey, syrup). Use food processor for convenience. There is similar way of preserving sorrel & other herbs with salt.
Frankly I think ordinary cooked jams are a bit of waste of time – they don’t have any health benefits. Although we do apple + aronia (black rowan) or apple + burberries jam or plum jam when we have to much of fruit.