When you take off at night lay it down crown up (on its side). This way you can compensate a bit as the movement will speed a bit in this position. Completely normal with mechanical movements. Even genuines.
I service all my watches and they are running strong. One A7750 is running constantly for 5 years as I wear it almost daily. The Asian movements are very reliable after a proper cleaning and service.
Hate to contradict you M but all A7750's I have ever seen have a special date advance wheel which makes it impossible to mess up. This opposed to the ETA 7750 which does have this problem. So in that regard the design of the A7750 is better than the ETA.
So my guess is the problem lies not with the date advance gear but setting lever or yoke instead. Probably just something out of place. Could even be the hack causing problems.
You can't upgrade 21.600 to 28.800 by replacing the escape wheel and/or balance.
If you want to upgrade you have to drop in an entire new movement and modify the hands.