Your fermentation technique is very similar to mine! I was actually talking about adjusting the hydrometer reading for temperature. If the temp was above what your hydrometer was calibrated at, the reading will be lower than it really is.
Malto-dextrines are unfermentable sugars (well up to 5% fermentable). They mostly add body and mouthfeel to the beer, but will add sweetness in bigger quantities.
If you add this, calculate how much you need to raise the beer the number of gravity points you want. Then you would dissolve that amount in ADDITION to the priming sugar solution.
Basically you would still prime the beer with sugar as usual, but also add malto dextrine to the solution. (Sorry, I couldn't figure out a clear way to explain that haha).
If it is sweetness you are after, there are many other options to bump up the FG that way (lactose, artificial sweetener, etc). Same rules apply!