Yes, this can be a problem, the start up fill is longer and is about 1/4 cup in the nano setting but this can typically be worked around by adding a drip irrigation valve to the end of the hose to slow the flow even more. A UPS could solve this in this power scenario and would not need to be more than the most basic small unit.
The Osmolator 2 you have seen is the replacement for the Nano Osmolator, we have finally gotten a delivery date for all the electronics and it looks like it should be launching around May 2023. It won't be as limited in tank size and my understanding is it will handle 10-150 gallon tanks, there will be a much more sophisticated version launching later to replace the regular Osmolator. For a considerable time both the old and new units will be produced as supply issues will persist for a while on electronics and some people will always prefer a more tried and true solution.