All suggestions are welcome mate :)
I have not calculated the water going back to the sump when the return pump is turned off. I think I will leave that to the sump installation then will adjust the overflow's top accordingly.
Yes, the overflow is connected to a pipe going towards the drain but about 4" off the ground outside the house.
The bio media requires really clean water. I have found with filter socks, very small particles would still get through and that is why I have another layer of filter wool/floss.
The last 2 drops can make the splashing noise but I think it's not as bad as the skimmer's noise.
Hopefully, this will eliminate the need to skim the surface and there is no stagnant water in the sump.