My observations on that matter:
1. Spawning gets messy after Mars is liberated, but this is actually for good, cuz you can get all the remaining convoy missions (usually, it's Oasis and Eos) using only two spawn areas:
- the road between Hendrix Hall and Mount Vogel exit in Eos;
- housing complex in the S part of Dust that gets attacked by EDF twice.
Sometimes the missions will spawn too far (like, in Dust while you are somewhere in Eos), but even so you can catch up with the convoy if your car is fast enough. Only that there will be very little time left for destruction itself.
2. Convoy missions start repeating (and spawning in the correct areas) only when ALL convoy missions are done. Correct me if I'm wrong.
3. The driver of the vehicle that has to be hijacked is usually persistent as hell and will not stop even after a direct hit from rocket launcher. But if some dumb NPC will drive into a narrow way and stop there, the entire convoy will stop and patiently wait until the path is clear