Hi seedpod, you are doing well to have plenty of well rotted manure and putting about a couple of inches on top of the soil would be about right, since you have already added quite a lot. I reckon that once soil fertility has been improved by a large initial dressing – which can be applied on the surface – the subsequent annual dose of one to two inches on top is about right to maintain health and soil life. I find that my clay soil resists drought much better from having the organic matter on top, making a softer and more crumbly surface.
Re acidity (see the lime question too) I would be surprised if the manure lowers your pH significantly and 6.5 is certainly good for vegetable growing.