A gate valve will give finer control, but I don't think that's the problem. It sounds like you may have a sump volume issue?
What's happening is normal, when you pump goes on the water level in the display will rise some, and the sump level will drop an equal Volume and then stay that way until the pump is shut off. The overflow only skims water when the level in the display rises that little bit. That's how it supposed to work so that tank doesn't empty when the power goes out.
If the sump level drops too much in this process then you may have a too small sump, or too strong pump (or the pump needs to be slowed with that valve).
Try this, with the pump off, fill the sump almost all the way, then turn on the pump and adjust the valve so that the level in the sump drops most of the way to the point where the pump starts sucking air. That's the most flow your sump volume cna handle, and that should work unless you need to completely choke the pump off to acheive that balance in which case a smaller pump is probably in order.