I've also had the same questions about the frequency of air patrols in GWX.
On my second patrol in a Type II "Canoe" along the East coast of Scotland, just off the Firth of Forth in grid squares AN01/47/51/52, I encountered air patrols every day for a couple of weeks, from 29 Sep through 12 Oct '39.
After getting a new Type VIIB and making several more patrols to the Western Approaches and Rockall Bank, up through the middle of 1940, I didn't see a single airplane.
I've found air patrols set up to appear from the Campaign folder as Scripted units and Random units. The files controlling aircraft strikes are in the "data\Cfg\Airstrike" folder. I haven't yet dared to try making changes to add more Scripted or Random air patrols. But, I have done some "tinkering" with the "Airstrike" file, comparing values there in the Stock game, in GWX and in LSH3. Some changes didn't seem to produce results, while other changes produced swarms of aircraft. I'm still trying to find a happy medium in all of that.
Lacking any good advice coming here from some of the experts, you might want to try experimenting with the settings in the "Airstrike" file to see if that gives you the "challenge" you're looking for.
Good luck...
