Berries need acid soil. Indeed the ph-value should be about 5.5. You can reach that value by filling the raised bed with lose garden soil and then a 30-40 cm layer of a 1:1 mixture of sand and peat (white peat preferred). A few shovels of loam are welcome as well. Instead of peat you also can mix in bark mulch.
Especially when you use calcareous irrigation water you should add peat und bark mulch to keep the ph-value as required. Otherwise the soil could get neutral or even calcareous and that could hinder growth. Also, use special berry fertiliser or other sour acting fertilisers. As you suggested you can also add some magnesium sulphate but the fertiliser plus the micronutrients are much more important.