I don't think there's anything you can add to your soil that will change the flavor of fruit, Dee. I suggest you have a soil test done—tell them you are growing berries when you do so—and the report should come back telling you what to add to the soil to make it optimal for the fruit. That's about all you can do. Sometimes a little water stress will improve fruit's flavor by concentrating them. And the flavor of fruit often improves as the plant matures, so maybe yours just haven't reached their prime yet.
I live in Dayton Or. Dee and we grow strawberries. The best way to get sweet berries anywhere is to leave them on the vine for as long as possible to ripen. It is the cool nights and warm days, length on the vine that helps to create sweet berries. This was not a good year in Oregon for berries so don't beat your self up over it. If to put them in the place you get the most sun, you might try to sweeten your soil with Agricultural lime. You can find it at Bimart for a fairly cheap price. Put it on in the fall and work it into the soil if you can or sprinkle it on to the soil around the plants and water it in. Good luck!!
Your painting above is fantastically gorgeous!! I assume you paint for a living on the coast. If you don't you should!!
Soil condition is the key. Adding compost helps. Some berries like acidic soil, especially blueberries. You can add a ready made compost that is higher in acidity, like one made for azaleas or a small handful or two of soil sulfur before growing season starts and several times during the year.
Potassium will make your berries sweeter.