Fence and tethered ballons with sensor packages underneath every few miles.

You have issues with weather affecting coverage but it would be a lot cheaper than a massive ineffective wall.

So now we have hundreds of monitors checking wobbly camera images all the time trying to spot tiny drones in all weather and all times of day. How often do balloons need refilling? Aren’t balloons highly visible and extremely vulnerable? Has anything like this ever even been tried in practice?

Been tried and working for decades.

https://en.wikipedia.org/wiki/Tethered_Aerostat_Radar_System

That's a high altitude aircraft defence system. We're discussing low altitude terrain level drones the size of a shoebox operating up to maybe 100m altitude, just enough to pop up over the wall. Air defence radar like that is going to be completely useless for this.