Your pH is fine. Bettas are able to adapt to a wide variety of pH levels. A stable pH is more important than and ideal pH. By using pH buffer products you can cause fluctuations that will end up hurting your betta.
It is very normal for your betta to just float around late at night if its dark. they just chill out. mine like to float in between the tank wall and the heater cable for stability, or literally tucked in between leaves of one of the plants