I won't worry about tannin extraction.
The most important thing is not the gravity, it is the % of wort that is fermentable sugars. Unconverted starches aren't fermentable, nor are dextrins (at least, not by your typical beer yeast). If as said you gradually rose through the mash temp range, you probably got enough conversion that you shouldn't have to worry about it making a decent beer. It may be a bit maltier and/or cloudier than normal, but it ought to be drinkable.