I've done this before with arrays, when a runway becomes available you push the number to an array. I set the variable to always choose something that is available inside the array so set the number to array.at(random(0, array.width)) will pick a random value that exists, such as 0,1 0,2 0,3 0,4 in the array.
So at the start you push 2 and 3 to the array. These would be at 0,1 and 0,2. When you come across 7 you push that to the array so 0,3 is 7 and the array looks like this (2,3,7). Then using array.at(random(0,array.width)) it will set the variable to a random number in the array, either 2, 3 or 7. Hopefully it's easy to understand the approach I was taking.