(First post here on SAS)
Hi!
There are a couple of tools that can help you, and achieve some of the "trigger" logic. They come from the Command & Control mod.
Check out the C&C_MissionTools type in the Full Mission Builder, you should get what you are looking for.
B.A.T book 03 (Operation sea lion: download link is on this post: (
https://www.sas1946.com/main/index.php/topic,56370.0.html look for batbooks 01-03)) has a good description of what these objects achieve, even though you might be able to find a more recent description of it.
I think the ones you might be the most interested in are the RandomPlanes ones, which remove aicraft. So you can achieve the randomisation of present aircraft with this by combining them.
You cannot modify the probabilities of the randomizers, but combining them will help. For example, placing two randomizers at the same spot will give the aicraft a 25% chance of appearance(50% chance twice).
But C&C is not limited to that, and has a lot more to offer, I encourage you to take a look at the different randomizers as well as the spotters.
There is one thing that I don't know how to achieve that the trigger achieve is the appearance of aircraft when entering the trigger zone. If there is a way, I don't know it.
Hope this helps!