Mixer in aeration tank
It all depends on the type of aeration system you are using. In my opinion, if it is properly sized to stir all the biomass, simultaneous mixing is useless and energy consuming.
It is perfectly normal for your mixer to work continuously. It is designed to optimize biomass stirring with oxygen and to avoid sludge settling.
Everything depends on the tank’s geometry and the aeration system.
If the aeration tank is a channel with blown air, it is imperative to mix at the same time as you aerate to improve diffusion.
In other circumstances, you can stop the mixer during aeration and restart it once the aeration has finished in order to improve denitrification.
I would further add that for fine-bubble aeration in a circular tank, a mixer placed along a wall allows for more homogenous aeration by driving the floc in a circular motion.
It is therefore stopped in the rest phase (denitrification).
It depends if your RAS pumps can handle hydraulic load. In the past, I've had to add to pumps solenoid timers, to reduce the hydraulic load!