Obviously, the air in the drainpipe can get in only one way.
And that is from the top, sucked in with the falling water from the overflow. All bubbles trapped in the falling water volume can obviously get out only at the bottom end that is submerged.
The only way to keep up and not change the drain speed you should split up the route it follows into 2 sections and let the air escape halfway down in the drainpipe. I'll make you a cross-section diagram to get the picture of what you should do because this kind of physics is difficult to explain in words only for people that don't yet see the obvious. Once you see it, it can't be unseen and becomes clear.
View attachment 174792
Size both tubes according to your turnover... If you hear water running noise coming from the open connection, plug it with a piece of sponge. Then air still can get in and out but the sponge will dampen or most likely eliminate the noise. This way the airbublles in the fall/stand pipe from the overflow escapes above the sump and not in it.
And you'll break the fall with a 90° horizontal gutter. The gutter should be a PVC pipe wider than the fall pipe in it. For example, 32mm fall pipe loosely placed in a 40mm or 50mm S-shaped pipe that is the open gutter to and in the sump.
How you need to size it depends on the capacity it needs to sync with your turnover.