No doubt someone else has thought of it, so credit to you too!
Instead of bothering with zones, trying to tweak enough to make passes happen when on the same tyres but then having it massively easy on fresher tyres, or putting it in a good place only to have a slow preceding corner space the cars out too much, why not just simplify the whole thing?
The idea here would be to let anyone within a second use their DRS for all of the time. The detection point would be at a place where the cars are mostly likely to be closets to each other, because even getting down under 1.5s you start to feel the dirty air.
But won’t it be too easy? Here’s the crucial part: reduce the angle at which the DRS can open, so the benefit becomes constant but small. That way the cars can follow each other throughout the whole lap and overtaking will be down to driver skill (and a tiny bit of boost down a straight).
There’d need to be some more tweaking and certain circuits may present a specific obstacle that would have to be allowed for (say, a bigger angle at Monaco because bits of the lap are corners separated by a tiny straight and there’d be no time to open it up), but the principle is what’s important here.