Based on your recipe (assuming 5 gal batch), I'd expect this brew to start at 1.071-1.072 and attenuate to about 1.012-1.013, so to me you're right on the numbers. But I also agree (as others have said) that 20% of your grain bill being dextrose (or any base sugar) is crazy high for an IPA, and could lead to the sour ("cidery"?) flavor and watery mouthfeel you've experienced.
On the other hand, could the "sour" flavor just be "yeastiness"? 1 or 2 weeks in the bottle and it can still have that young, green, "overly yeasty" flavor.
I see it's now at 4 weeks in the bottle and getting better. Does it seem to be getting clearer? I.e., yeasty matter dropping out of suspension? I bet it will be even better at 6-8 weeks in the bottle. And your next batch, where you don't go over about 5-10% dextrose in the grain bill, will be better still!