what is happening is you are getting a vapor lock and it's causing it to restrict the flow.
Increasing the hole size to at least 1" is also advisable.
You don't need
an overflow box, bot it makes life sooooooo much easier and avoids some potential problems.
My system is drilled through the back and I have a T connector so they is a free air passage;
If I plug the open end the drain is decreased by half easily. The open end needs to be above the water level (of course) and I capped mine, drilled a hole in the cap and glued in a air line valve so I can control the amount of air being drawn through. Don't ask me to explain all the physics involved, but I know it works.
I don't recall the name/type of overflow/drain style it is fashioned after, but it works.
I have a bulkhead going through the tank into the overflow box.
My drain plumbing is 1" and the return side is 3/4" using a DC
pump so I can easily control the flow rate without using valves.
hope that helps, but as Sniper said, research more before doing any further modifications.