Submersible Pump for Water Storage Tank?

External pump's fine.

You have a wide choice.

You can use a centrifugal pump unit (maybe self priming) but then you have to do some changes to the piping. I don't think the total price will be less than repairing the submersible.

Replacing with another submersible would be the easiest.

Whatever you do you will have to include water level sensors to stop the pump when the water is low. This can be float switches or electrodes.

Centrifugal pump will work, but you need an outlet at the bottom of the tank so that you have adequate net positive suction head (NPSH) for the pump.

Or, you might get away with running the pickup up and over the top of the tank (don't know how tall it is)to the pump, but the pump should be as close to the ground as possible. This might require priming the pump on start-up, but if your plumbing is leak free, you should only have to do this once.

Your existing control scheme should work, but a low level shut-off switch is in order to protect the pump.

Good luck.