An3s

I'm not sure if the following is legal in terms of keeping municipal and rainwater/greywater separate but it's something I'd do without it bothering my conscience.
If you want to make the change over system automatic you could install a ball float valve (like the ones in your toilet cistern) inside and close to the bottom of your rainwater tank and connect it to the municipal supply. When the rain water level in the tank drops to say the 25% capacity the ball float valve will open and maintain the level at 25% using municipal water. Just determine where the crossover point needs to be by moving the ball float valve up or down inside the tank.
Your booster pump will never run dry unless there is no rain water and the municipal supply is off and you'll never need to worry about manual intervention. Alternatively install a float switch in the tank so that the booster pump can't run when the tank is below a certain level.
The ball float valve prevents rain water from feeding back into the municipal supply although the municipal pressure will almost always be higher than the head of water in the tank (except when there are water outages or if the tank is on a very high tank stand in which case you wouldn't need a booster pump). Regardless it's not possible to accidentally feed rainwater back into the municipal supply using a decent float ball valve.
N.B. Just make sure that the outlet from the ball float valve is in clear air (not underwater) when the valve starts to open. This prevents siphoning/feedback into the municipal supply when the municipal supply is off and the level in your tank is in the "top up" area. You can do this by bending the float arm downwards or installing the valve at a tilted angle so the float is lower down than usual.