The level of the overflow is how you control the water level in the main tank. With an unbroken siphon, you either have to perfectly tune the siphon to the return (I wish you the best of luck!) or seal the sump airtight.
Some people use a siphon with an external overflow box. This method combines the best of both worlds, but requires extra equipment and space between the rear of the tank and the wall.
Some have also used a pump that outpaces the drain and used a float switch to cycle it. This works under normal operation, but has no safety margin should there be a power loss. With a siphon, you will either overflow the sump or break siphon, or both if the power goes out, or the siphon outpaces the return pump.