If the volume of the vertical column was to slowly drain back into the pump and trigger it back on, you would have an endless cycle of the pump repeatedly running. Obviously, that's not a good setup.

If your concern is the water at the top of the vertical pipe freezing in the attic, and the garage doesn't get below freezing temps during the winter, I wouldn't worry about it. You've said that you insulated the attic piping, so I assume the top of the standing water column is also insulated. This certainly helps, but heat will also be provided from the water below; as the top water does cool, convection loops will be set up within the pipe with the warmer water below and in this way heat from below will be transferred to the top of the water column keeping it from freezing.

If the garage does freeze in the winter, you can always install a self-regulating heat tape to the piping holding the water. These tapes are made with a temperature sensitive semiconductor-based material between the conductors such that only when the temp drops does the material begin heating, and when the heating becomes sufficient, they stop increasing the heat. Don't even consider the old style heat tape with a separate thermostat, the new tapes are much safer (the tapes will not overheat in open air, something the old tapes would do if not held in intimate contact with piping).

This is just on example of a semiconductor heat tape:

https://www.briskheat.com/products/heaters/heating-cable/self-regulating-heating-cable/speedtrace-pre-terminated-self-regulating-heating-cable/speedtrace-self-regulating-heating-cable-1.html